Well, the simple answer to your first question is "ingredients & technique." Once dissolved & boiled, sugar wants to return to it’s crystalline structure; you want to coerce it to not.
Starting with ingredients, your recipe will usually have something to push between the sugar molecules to keep them from aligning into crystals; either a different type of sugar (corn syrup, honey, maple syrup), or an acid (lemon juice, vinegar, cream of tartar), which will invert some of your granulated sugar so that the new invert sugar will interfere. Marshmallow works really well because it has syrup plus gelatin & egg whites, which coat sugar molecules. Even with interference, you still to handle it specially. As a sugar syrup cools, if crystals do form, they will act as seed crystals for the rest of the batch & give you a large grain (sugar really wants to go home to crystal). Agitation, rough surface, and seed crystals can be a problem.
Rough areas can start the syrup crystallizing, so make sure your pan is smooth inside. My grandmother also used to butter the inside of her pan to keep the crystals from growing up the side during boiling, then falling back in & acting as seeds. Some candy recipes say to put a lid on the pan for 3 minutes when it boils - that gets condensation to wash down the sides, but check your recipe - some fudge ingredients will boil over. Don’t move your candy thermometer around and if you have to take it out & put it back in, wash the syrup off with hot water, same with your stirring spoon (seed crystals again). Different recipes call for different techniques, but the ones I've seen say dissolve sugar before boiling, and don’t stir or agitate after the mix boils. Let it boil to the temp. your recipe calls for - different ingredients change the soft-boil point. Then gently take it off the burner & dip pan base into cool water. Some recipes call for beating it immediately for larger grain; for a finer grain, keep pan in water until it cools to 110f., then start beating until it thickens.
Ready for a recipe? Once my grandma discovered marshmallow fudge, she never made anything else, but one of her cookbooks was "Candy Making" from the "Woman’s Institute of Domestic Arts and Sciences, Inc." cc 1918, 1919, 1929. That should be old-fashioned enough :lips: I’ve copied it exactly:
The making of fudge, in which brown sugar is used for the largest part of the sweetening, is explained in the accompanying recipe. Peanuts are added, but if desired, nuts of any other kind may be used.
2 C brown sugar1 Tb. Butter
1 C white sugar1 tsp. Vanilla
1C milk¾ C chopped peanuts
Mix the sugar, milk, and butter and boil until a soft ball will form in cold water, or a temperature of 238 degrees is reached on the thermometer.
Remove from the fire, add the vanilla, and cool until the heat is out of the mixture.
Beat and, when the candy begins to grow creamy, add the chopped nuts.
When sufficiently thick, pour into a buttered pan, cut, and serve.
Also, if you see any recipe called "penuche, penuchi, or penuchy," that will have brown sugar in it. Can’t find the reference, but I believe "penuche" was a word for a particular type of brown sugar but now includes any kind.