Preheat oven to 350℉ and line an 8- or 9-inch square baking pan with parchment paper, or coat with non-stick spray.
Melt butter in a medium-sized sauce pan, then remove from heat.
Add cocoa powder and stir well.
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, being careful not to overbake. 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.