Preheat the oven to 350 degrees F
Grease an 8x8-inch pan
Melt butter in a microwave safe dish in the microwave
Combine the melted butter, sugar, cocoa powder, vanilla, eggs, flour, baking powder, and salt
Stir together until just combined and no dry ingredients are left
Add nuts or chocolate chips if desired and gently stir to mix in
Spread the batter into the prepared pan
Bake in preheated oven for 20 to 30 minutes (until desired doneness)