Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan or line it with parchment paper.
In a medium saucepan, melt the butter over low heat. Once melted, remove from heat and stir in the sugar until well combined.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, sift together the cocoa powder, flour, salt, and baking powder. Gradually add this dry mixture to the wet ingredients, stirring until just combined. Be careful not to overmix.
If using, fold in the chocolate chips or chunks and chopped nuts until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it out evenly.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (but not wet batter).