01 -
Line an 8-inch square pan with parchment paper, leaving overhang on two sides. Lightly butter the parchment and set the pan near the stove.
02 -
Whisk the sugar, cocoa powder, and salt in a heavy 3-quart saucepan. Add the milk and corn syrup, then stir over medium heat until the mixture is smooth and the sugar begins to dissolve.
03 -
Clip a candy thermometer to the pan. Bring the mixture to a steady boil and stop stirring once it boils. Cook until the thermometer reads 234-236°F.
04 -
Remove the pan from the heat. Place the butter pieces on top and add the vanilla, but do not stir. Let the mixture sit undisturbed.
05 -
Cool the mixture at room temperature until it drops to about 110°F. This can take 45-60 minutes depending on the pan and room temperature.
06 -
Beat the fudge with a wooden spoon until it thickens, lightens slightly, and loses its glossy shine, about 5-8 minutes by hand. Fold in nuts if using.
07 -
Quickly spread the fudge into the prepared pan. Let it set at room temperature for 1-2 hours, then lift it out and cut into 36 small squares.