Preheat oven to 350 degrees and line a cookie sheet with parchment paper
In a medium bowl, combine the flour. cocoa powder, baking soda, and salt
Using a standing mixer, cream together the butter, sugar's and peanut butter until smooth and creamy
Mix in the egg and vanilla and mix until combined
Stir in the dry ingredients
Mix in the Reese Piece candy
Using a small ice cream scoop, scoop out some dough and roll it into a ball
Place onto the cookie sheet and slightly push down with the bottom of a glass cup
Bake in the oven for 10-12 minutes
Let cool completely while making the frosting