How to Fix Common Fudge Problems
from https://www.thespruceeats.com/, by Elizabeth LaBau

Traditional fudge, old-fashioned fudge, is actually a close relative of fondant. It shares the same cooking process and theory: a sugar syrup is boiled, then cooled to a specific temperature, then beaten just until it starts to crystallize. When done properly, this type of fudge has a texture that cannot be imitated; firm creamy, and smooth, it’s an absolute pleasure to taste. But old-fashioned fudge isn’t the only game in town. "No-fail" fudge recipes have become more popular. These shortcut fudges use chocolate and marshmallows to help the fudge set and give it a smooth texture, eliminating the need for long periods of cooling and beating.

If you're having trouble with your fudge, you're probably following a recipe for old-fashioned fudge—candy cooked to a specific temperature, cooled, then beaten until thick. Old-fashioned fudge can be fussy, especially on your first or second attempt. Some home cooks complain that their fudge is grainy, crumbly, or burnt while others complain that their fudge never sets properly. If you've encountered one of these problems, don't worry—you can probably save your fudge.

Fixing Fudge

Sometimes old-fashioned fudge never sets, even after hours in the refrigerator. You wait patiently, only to discover that it's still a sticky, gummy mess. But don't despair or throw out the entire pan of fudge: You can probably remedy the situation.

Fudge usually behaves this way when it's not cooked to a high enough temperature (due to oversight or a faulty candy thermometer).

If your fudge is tough, hard, or grainy, then you may have made one of several mistakes: You may have overcooked it, beaten it too long, or neglected to cool it to the proper temperature. Don't throw out the whole pan, because you may be able to melt the fudge down and try again. Of course, if your fudge has a distinctly burnt or scorched flavor, you'll have to start over with a fresh batch.

To fix soft fudge or hard fudge, simply follow these easy steps:

• Scrape the fudge back into a large saucepan and add 1 1/2 cups of water.
• Stir the fudge over low heat until it dissolves. Carefully taste the mixture, as the water probably diluted the flavor. Add more flavorings if necessary.
• Increase the heat to medium and bring it to a boil, washing down the sides of the pan frequently with a wet pastry brush to prevent sugar crystals from forming. Do not stir the fudge.
• Cook it to the proper temperature specified in the recipe (most likely between 237 F and 239 F).
• Take it off the heat, and follow the recipe's instructions for cooling and beating the fudge. As you beat the candy, remember that the mixture should lose its sheen and thicken before you pour it into the pan.

Tips for Fudge Makers

• Before you make another batch of fudge, it's a good idea to test your candy thermometer. Place it in boiling water to make sure that it registers 212 F. If it doesn't, you should calibrate it or invest in a new one. Many people overcook fudge because of faulty or broken thermometers.
• Once the sugar has dissolved and the mixture has come to a boil, do not stir it. If you do, the sugar can crystallize, giving your fudge a gritty texture.
• Avoid stirring or agitating the fudge whenever you are not explicitly instructed to do so. It is especially important to let the fudge rest, undisturbed, during the cooling process, and to let it cool all the way to the specified temperature before stirring. Stirring the fudge when it should be cooling, or even vigorously jostling the pan, is a recipe for crystallization.
• Fudge that is too hard or too soft is the result of two factors: the cooking temperature and the amount of stirring it receives. Cooking the fudge even two or three degrees higher or lower than the temperature specified can be the difference between a successful candy and a soupy mess or a rock-hard slab of sugar.
• Likewise, there is a fine line between stirring the fudge too much and too little. If you do not stir it enough, the fudge will never set. If you stir it too much, the fudge will harden and set before it can be scraped into the pan. As you beat the fudge, pay attention to color and texture. Once the fudge loses its sheen and thickens, goes from shiny to matte, put down your spoon. If you continue to beat the fudge, it will go from “perfect” to “rock hard” in minutes.
• If you're making a lighter fudge that doesn't involve any chocolate, you might notice that the recooked batch has a darker, brownish color, thanks to caramelized sugar crystals. This change may alarm you, but the fudge should still have a lovely, mellow flavor and silky texture.