Layer in a 1-quart glass jar or canister the following ingredients: cocoa powder, sugar, chocolate pieces, flour, baking powder, salt, walnuts, and white baking pieces. Tap jar gently on the counter to settle each layer before adding the next. Cover jar.
Store at room temperature up to one month. Or, attach baking directions and give as a gift.
Heat oven to 350 degrees F. Grease and flour an 8x8x2-inch baking pan. Combine butter and eggs in a large bowl. Stir in jar contents. Spread into prepared pan. Bake for 35 minutes or until edges begin to pull away from pan. Cool in pan on a wire rack. Cut into bars.