Preheat the oven to 350 degrees F (175 degrees C).
2
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.
3
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.
