Preheat oven to 350° F. Coat a 9-inch-square baking pan with unsalted butter or baking spray.
Line the bottom and 2 sides of the pan with parchment paper cut to fit, leaving an overhang on the 2 sides. These overhangs will serve as 'handles' for easily removing the cake after the bake.
In the bowl of your stand mixer again, this time fitted with the paddle attachment, cake flour, sugar, cocoa, baking soda, and baking powder for 1 min on medium-low speed.