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.