Add the 1 cup of butter (2 sticks) to a saucepan. Put the saucepan on low heat and melt the butter fully. Raise the heat to medium and stir constantly. Let the butter foam slightly. When it turns a rich brown color remove it from the heat and whisk until the butter turns dark brown.
Divide the butter evenly into two large bowls. Since moisture has cooked off during the browning process, each bowl should contain about 100 grams of butter.
Add 1 tablespoon of milk to each bowl and whisk to temper the butter. Let it cool for a few more minutes.
Whisk one egg into each bowl. Then add the sugars and whisk again.
For the chocolate batter, add in the flour, cocoa powder, baking powder and salt to one bowl. Stir to combine.
For the peanut butter, whisk in the peanut butter to the second bowl, then add the flour, baking powder and salt. Stir to combine.
Split the chocolate chips evenly between the cookies.
Scoop out 3 tablespoons of the dough. Roll the dough together to create 10 large "marble" balls.
Place the cookie dough balls on a parchment-lined baking sheet and chill in the refrigerator while you preheat the oven to 350°F.
Bake for 10-12 minutes at 350°F. The key is to remove the cookies before they look fully done. They will continue to "cook" outside of the oven, so this method helps them maintain the chewy texture.
Let cool and serve!