I've made plenty of batches of chocolate fudge and all of a sudden, the cocoa butter separates out and the fudge won't stiffen when I stir it up.

I'm using the same brands of everything and the same recipe.
3 cups gran. sugar - Domino
1 1/2 cup heavy cream - Schepps
1/4 cup light corn syrup - Blackburn's
1/4 tsp salt - Morton
6 oz. semi sweet chips - Nestle

I calibrated my thermometer. I heat the mixture without the chocolate to 240 F and test it in cold water to make sure it's at the soft ball stage.

I pour it in a clean, stainless steel bowl with the thermometer and let it cool to 110 F. It takes about two hours and I set the timer so I'll be there when it's at the right temperature.

Then I stir it up and as soon as it lightens, it starts to turn stiff and then the cocoa butter comes out of the mix. Sometimes it will still set, but it loses its consistency and it looks awful

I checked and double checked everything.

The last thing I found was that if I poured the 240 F mix over cold (refrigerator) chocolate, that may cause it to separate.

I used to pour the chips into the bowl and leave them there while the mix heated. This took about twenty minutes.

Is that the reason it separates? The kitchen is air conditioned to 74 F, though it's obviously a little hotter when the stove is on. I had the same problem on rainy and dry days.

Humidity?

I have had fudge seize up before with the accidental introduction of even a little moisture.

A drop or 2 of oil will keep it from separating. I am not a pastry chef but I am thinking that the fat content of the Nestles morsels is pretty low. The cocoa butter is removed and sold as a cash side stream. So the company will add only enough to allow the chocolate to liquify under friction/mixing. In a commercial setting high fat content chocolate is used. We used to buy Guittard in large bars which worked real well for making ganache PITA to portion but worked well.

It doesn't seize. It gets oily after I stir it for 3 to 5 minutes and the oil - cocoa butter - will not blend back into the mix. I thought about adding a few drops of water to see if forcing it to seize might work.

Possibly, but I made several successful batches of it with Nestle in the past few months. I've been wondering if they changed the formula.

I checked their information and compared it to Giardelli.

Nestle:
28 gram serving, 6.9 grams total fat, 3.9 grams saturated fat

Giardelli used a 15 gram serving, so I multiplied their numbers by 1.867.
28 gram serving, 8.4 grams total fat, 4.6 grams saturated fat

I started putting a tablespoon of unsalted butter in the mix, but I left that out when I first had problems getting it to set. Since then, I found my thermometer was miscalibrated. I adjusted it, but I left the butter out. I'll try that next.

It was the fucking bowl.

You beat the fudge at 110 F, after it was heated to 240 F. Sugar slowly decomposes (it does not melt - sucrose decomposes into glucose, fructose, and other compounds involved in carmelization), but these aren't the reason the fudge turns shiny when you beat it.

It's milk fat.

First, it's a shiny, sticky mass. As you beat it and the milk fat separates, the mass becomes slippery. You can test this easily if you use a plastic spatula to scrape the side of the bowl while you beat it. I do that to ensure that the flavor mixes evenly. After a couple of minutes, the fudge isn't set, but it won't stick to the spatula. It slides off.

Stir it for a few more minutes and it starts to form the fine crystals that make it fudge. It changes color at this point. It gets lighter. The last stage is when the milk fat is re-absorbed into the sugar crystals. It turns dull and it's ready to mold it in the pan. This takes five to ten minutes.

I used a clear Pyrex bowl to make the early batches. You have to sanitize your equipment if you want to sell your food, so I bought a stainless steel bowl to distinguish it from the personal use one that I wouldn't bother to sanitize.

The chocolate also has a load of cocoa butter and the milk fat would not set up in the steel bowl. I could see drops of it sprayed on the side of the bowl, although it worked with vanilla, strawberry, and grape. The more I mixed it, the more it separated. This thread documents how much I fucked with it to try to understand what happened.

Last week, I had changed and tested everything and I thought of the glass bowl. What the Hell, it's the only thing left.

That was it.

Mastering chocolate fudge:

3 cups sugar
1 1/4 cups heavy cream
1/4 cup cornstarch
1/4 teaspoon salt
3/4 cup semi-sweet chocolate chips Nestle is fine; the chocolate wasn't the problem.

Mix all of the above in a bowl. Pour the mix into a 5 or 6 quart pot. Any smaller and it will overflow when it boils; any larger and it burns. Ask me how I know.

Heat the pot on low heat. I use setting 1 on the large burner on my electric stove. The dial indicates low, 1-9, high. Normally I cook the mix at 2.5, but the chocolate chips settle and burn.

Stir the mix every couple of minutes until it's homogenous. This will happen around 160 F. I stir it with a plastic spatula that I got from Bed Bath & Beyond. They're rated to 450 F. I melted a spatula from the supermarket in a pot of fudge, so I learned to check the temperature rating. They're also cheap. Four of them cost about six bucks.

Turn up the heat when the chocolate chips are all melted. Make sure there are no half melted chocolate chips on the bottom of the pan. If there are, they will burn when you raise the heat. I leave it alone until it gets to 230 F. Then I increase the heat to 3.5 and fill a bowl with water and a couple of ice cubes to test for the soft ball stage.

When the thermometer reads 240 F, quickly test it for soft ball, then pour it into a glass Pyrex bowl. NOT STAINLESS STEEL (SEE ABOVE POST.) I learned that the shape and texture of the reactor can affect a chemical reaction in my brief flirtation with chemistry. I threw about twenty pounds of it in the trash until I figured it out.

Another note on cooking: You can overheat fudge mix a couple of degrees and it will still work if it doesn't have chocolate in it. I let one batch heat to 244 F. It was vanilla fudge and it was fine.

I also overheated a batch of chocolate to that temperature. It turns into a giant Tootsie Roll. While the idea of a giant, bowl shaped Tootsie Roll may sound excellent in concept, it sucks in reality. Go buy a Tootsie Roll and try to break a piece of it off with a spoon. If you did this, the batch is junk. If you realize you did it, mix it with hot water and lots of grease breaking dish soap like Dawn. If you find out when it cools, fill a kettle and boil it. Pour some of the water into the bowl and you can scoop some of the mix. Lather, rinse, repeat.

Setting the fudge:
Let it cool to 110 F in the Pyrex bowl. This takes about 2:15, depending on the ambient temperature and the particular mood of the chocolate gods that day. The last ten degree take forever. You can stir it by hand if you want. I use a handheld mixer with the bread blades. It may set in three minutes. It may set in thirteen minutes.

Fudge sets in stages.
1) First it's a bowl of dark, sticky goo. If you put a spoon in it, it sticks to the spoon.
2) After about five minutes, you'll notice little swirls of lighter color. They'll disappear quickly.
3) Then it turns glossy - much more shiny than it was before you mixed it. The gloss is milk fat.
4) After a minute or two of this stage, it become very slick. I use a spatula to scrape the bowl and the blob drops won't stick at this stage.The spatula is clean.
5) In another minute or two, the color lightens. This is very noticeable. You won't have any doubt when it happens.
6) Then it becomes stiff. It's still glossy, but it forms peaks that persist if you leave them alone. Fudge is extremely fine sugar crystals with flavor in it. That's all it is. I think the color and texture change happen because the crystals have begun to form.
7) The final stage is when it turns to a matte finish.  With vanilla or other flavors, this takes a couple of minutes. With chocolate fudge, it takes maybe thirty seconds. At this point, I think the crystal structure absorbs the milk fat. When this happens with chocolate, try to get it even and then scoop it into the pan. If you keep mixing it, it thickens and it's impossible to mold it into the shape of the pan.

This process takes between three and fifteen minutes.

Want a better answer than that?
Y'ain't gonna git one. India isolated and refined sugar around 500 A.D. Your mom told you that when you heat sugar, it melts. We've been cooking with sugar for one thousand and five hundred years and in 2011, a professor of food chemistry named Shelly J. Schmidt determined that sugar does not melt when you cook it. Your mom lied.

I just confirmed this with the guy at Ace Restaurant Supply in Haltom City. His wife makes chocolate fudge every year for Thanksgiving. She uses a Pyrex serving dish to beat it because it will not set in a stainless steel bowl. Videos of fudge makers show them cooking in a copper pot and beating the fudge on a marble slab.