Preheat the oven to 350°F (175°C) and grease a 9 x 13-inch baking pan.

Mix melted butter and cocoa in a large bowl. Add the flour and sugar, mixing until combined.

Add the eggs and vanilla, mixing well. Fold in the chopped nuts, if using.

Pour the batter into the prepared pan and bake for 20-25 minutes. Check with a toothpick—it should come out clean.