Melt the butter in a small saucepan over medium heat until completely melted. Turn off the heat and let the butter cool to room temperature, 1 - 2 hours. Transferring the melted butter to the stand mixer bowl should help it cool faster.

Add brown sugar to the cooled butter and beat with the paddle attachment for 2 minutes until thoroughly combined.

Add the egg, egg yolk, milk and vanilla extract to a small bowl or measuring cup, whisk to combine and add to the mixture in the stand mixer bowl. Beat for 2 minutes until light and fluffy.

In another mixing bowl, sift together the flour, cocoa powder, baking soda, and baking powder. Stir in the kosher salt and give it a good mix to distribute. Add the flour mixture to the wet ingredient mixture in the stand mixer bowl, stir until incorporated. Stop the mixer and scrape down the sides and bottom of the bowl as necessary.

Add the chocolate chips and peanut butter chips, stir to incorporate. Stop the mixer and scrape down the bowl with a spatula and give it another good mix as needed.

Portion the cookie dough with a large cookie scoop and place it on a parchment lined baking sheet. Refrigerate for at least 1 hour before baking.

Preheat the oven to 350°F.

Place 6 dough balls on a baking sheet, about 2 inches apart and bake 1 batch at a time for 14 minutes.

Allow the cookies to cool on the baking sheet for 10 minutes before removing them to a wire rack to cool completely.