Preheat the oven to 350 degrees F (175 degrees C). Grease a 9×13-inch pan.
Combine the melted butter, sugar, cocoa powder, vanilla, eggs, flour, baking powder, and salt. Spread the batter into the prepared pan. Decorate with walnut halves, if desired.
Bake in preheated oven for 20 to 30 minutes or until a toothpick inserted in the center comes out with crumbs, not wet. Cool on wire rack.