Preheat oven to 350 degrees F (175 degrees C). Grease a baking sheet.
Whisk flour, cocoa powder, and baking soda together in a bowl. Beat butter, white sugar, brown sugar, egg, and vanilla extract together with an electric mixer in another bowl until smooth and creamy. Gradually beat flour mixture into butter mixture until dough is just incorporated. Spread dough out onto prepared baking sheet.
Bake in the preheated oven until set, about 14 minutes. Cool completely and crumble as desired.