Beat the butter and sugar with an electric mixer in a large bowl until smooth. Add the egg and beat until incorporated; then beat in the peanut butter until evenly mixed. Stir in the cake mix until just incorporated. Shape the dough into 1 inch balls, place onto ungreased baking sheets, and flatten with a fork.
Bake in the preheated oven until the edges are golden, about 10 minutes. Allow the cookies to cool on the baking sheet for 1 minute before removing to a wire rack to cool completely.
