2. Whisk Together the Flour, Cocoa Powder, Baking Soda, and Salt in a Medium Bowl. in the Bowl of a Stand Mixer Fitted with the Paddle Attachment, Beat Together the Butter, Shortening and Brown Sugar on Low Speed until Just Combined. Increase the Speed to Medium and Beat until Fluffy and Smooth, about 3 Minutes. Add the Egg and Vanilla and Beat for Another 2 Minutes