I like to coat the sides of the saucepan with butter or oil before I even start cooking any kind of candy because it helps prevent boil-overs and keeps sugar crystals from sticking to the sides.
When you’re transferring the warm fudge from the saucepan to the cooling pan, don’t scrape the sides or bottom of the saucepan or you may introduce unwanted sugar crystals into your finished fudge.
8 tablespoons unsalted butter
½ cup milk
½ cup granulated white sugar
½ cup brown sugar, firmly packed
Pinch of salt
1 teaspoon vanilla extract
½ cup unsweetened cocoa powder
2 cups confectioners’ sugar
Mix butter, milk, salt, brown and white sugars in a heavy pan on medium heat. Bring to a boil and stir continuously for 6 minutes.
Remove from heat and cool briefly, then add vanilla, cocoa powder, and confectioner's sugar. Beat with an electric mixer until fudge is smooth and thick.
Pour onto a buttered baking pan. When cool enough to touch, use a spatula to help roll mixture into log form, wrap it in plastic wrap and place in freezer for 20 minutes.