In a medium-sized bowl, blend together flour, baking powder, and salt.
In a large saucepan, melt butter. Remove from heat, then stir in cocoa powder until blended. Stir in sugar. Then stir in 4 eggs until thoroughly blended. Do not over mix. Add vanilla and mix again.
Blend in dry ingredients and nuts if using.
Pour batter into prepared baking pan. Smooth top.
Bake for approximately 30-35 minutes or until a toothpick inserted in the center comes out clean.
Cool and cut into squares.
Recipe by Just a Smidgen at https://justasmidgen.com/2011/11/01/a-promise-is-a-promise/