In a large mixing bowl, combine the melted butter, granulated sugar, and unsweetened cocoa powder. Stir until well combined.
Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the flour and salt to the wet ingredients, mixing just until combined. If using, fold in the chopped nuts.
Pour the batter into the prepared baking pan, spreading it evenly.