Preheat oven to 350°F. Line two cookie sheets with silicone baking mats or parchment paper.

Place butter and peanut butter in a large bowl. Using a hand or a stand mixer, mix until combined then add both sugars. Cream until smooth. Mix in egg, vanilla, melted chocolate, and milk and mix on medium speed until combined.

Mix in baking soda, cocoa, and salt, then slowly mix in flour and mix just until the batter is smooth and comes together. Be sure to scrape the sides of the bowl during mixing.

Scoop 2 tablespoon balls of dough and place 2-inches apart on prepared cookie sheets. Using a fork, create a criss cross pattern on the cookies to flatten slightly. (Tip: if the dough starts sticking to the fork, dip it in a bit of granulated sugar.)

Bake for 9-13 minutes, or until the edges are just golden brown and the tops are no longer glossy. Cool on cookie sheets at least 10 minutes before removing.