Preheat oven to 350 degrees. Coat an 8x8x2 square baking pan with cooking spray and line with parchment paper. In a mixer fitted with a paddle attachment, cream the butter and sugar for 10 minutes.
Meanwhile, in a separate bowl sift together the cocoa, flours, baking powder and salt. Add the eggs one at a time to the mixer followed by the vanilla, and beat for an additional 5 minutes. Gradually incorporate the dry ingredients into the mixer, alternating with sour cream and ending with the flour mixture.
Scoop the batter into the prepared pan and spread evenly with an off set spatula. Sprinkle with pecans if desired. Bake for 15 minutes, rotate, and bake for an additional 18 minutes. Let the brownies cool in the pan on a wire rack for 30-40 minutes.