Preheat oven to 350℉ and line an 8- or 9-inch square baking pan with parchment paper, or coat with non-stick spray.
Over low heat, melt butter in a medium-sized sauce pan. Remove from heat as soon as the butter melts.
Add cocoa powder and stir well. Ensure the mixture is barely warm before moving to the next step - you don't want to cook the eggs!
Mix in sugar, eggs, and vanilla until shiny and smooth.
Add flour, baking powder, and salt. Mix well.
Fold in walnuts and/or chocolate chips, if desired.
Pour batter into prepared pan and bake for 15-20 minutes. The brownies are done when the edges begin to pull away from the sides of the pan and a toothpick inserted in the center comes out clean.
Cool and cut into squares.