Preheat oven to 375 degrees. Line cookie/baking sheets with parchment paper.
In a large mixing bowl cream together butter, peanut butter, brown sugar, milk, and vanilla using an electric hand mixer. Add egg and mix until combined.
In a separate mixing bowl whisk or sift together flour, cocoa powder, baking soda, and salt. Add dry ingredients to the wet ingredients and stir together with a wooden spoon until all combined.
Scoop the dough using a 1 1/2 inch cookie scoop and place into bowl with sugar. Roll into balls and coat fully with the sugar. Place onto lined cookie sheets. Leave about 1 inch between cookie balls.
Using a fork make a crisscross on top of each cookie flattening them down.
Bake in preheated oven for 7-8 minutes until crispy round the edges and just set on the top of cookies. Remove from oven and allow to rest on pan for 5 minutes. Remove to a wire cooling rack to cool completely.
