Old-Fashioned Fudge

Mix 4 cups (800 g) sugar, 1 cup (84 g) Dutch-process cocoa powder, 1½ cups whole milk, 2 Tbsp. light corn syrup, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a large, heavy saucepan or Dutch oven fitted with a candy thermometer. Place over medium heat and cook, stirring often, until sugar is completely melted, 7–10 minutes. Increase heat to medium-high and allow the mixture to come to a boil. Cook until thermometer reads 234° F (soft-ball stage), 9–12 minutes. Remove from heat and stir in 6 Tbsp. unsalted butter and 2 tsp. vanilla extract. Let cool in the pot at room temperature without stirring until thermometer reads 110° F, 30–45 minutes. While the fudge is cooling, butter the bottom and sides of an 8" square pan and line with a strip of parchment paper that hangs over two sides to create a sling.

Beat fudge with a wooden spoon until it becomes very thick and loses its gloss (4–6 minutes of brisk beating; it’s okay to take quick, 30-second breaks as needed). You will know it’s ready when the fudge becomes very thick and starts to harden ever so slightly. Working very quickly, transfer fudge to prepared pan and press into an even layer using an offset spatula or clean hands. Top with chopped salted toasted nuts or flaky salt (if using); allow to cool uncovered at room temperature at least 4 hours, preferably overnight. Cut fudge with a sharp knife, dipping the knife in hot water and wiping with a towel between cuts if necessary. Do Ahead: Store fudge between layers of wax paper or parchment paper in an airtight container at room temperature up to 1 week.