Start by lining a 9x9-inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a medium saucepan over low heat, add the sliced unsalted butter and the block of American cheese. Stir continuously until melted and combined.
Remove from heat and gradually add powdered sugar, cocoa powder, and salt. Stir until well combined and no lumps remain.
Stir in the vanilla extract and fold in chopped nuts if using.
Pour the fudge mixture into the prepared baking pan, spreading it evenly. Let it cool for about 1 hour or refrigerate for quicker results.
Once set, lift it out of the pan using the parchment paper and cut into squares.
