Preheat oven to 350 degrees.
In a bowl, mix together flour, cocoa powder, baking soda, and salt. Set aside.
In the bowl of your stand mixer, cream together sugar, butter and peanut butter. Mix over medium speed until smooth. Beat in egg.
Add flour mixture to the wet ingredients and beat until smooth.
Drop rounded tablespoons of dough onto a parchment paper lined baking sheet. Gently press dough down with a fork. Sprinkle top with sugar.
Bake for 10 -11 minutes.
Transfer cookies to a cooling rack to let cool.