Preheat the oven to 375°F. Line two baking sheets with parchment paper; set aside.

In a large bowl, beat the butter, peanut butter, brown sugar, milk, and vanilla with an electric mixer at medium speed until well blended. Add egg. Beat just until combined.

In a small bowl, combine the flour, cocoa powder, salt, and baking soda until well blended. Add to the peanut butter mixture and mix at low speed just until combined.

Using a 1.5 tablespoon cookie scoop, portion the dough two inches apart onto the prepared baking sheets. Flatten slightly in a crisscross pattern with the tines of a fork.

Bake for 7-8 minutes or until set. Take care not to overbake.

Cool on the pan for 5 minutes then transfer to a wire rack to cool completely.