Preheat your oven to 350°F (175°C).
In a medium mixing bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and light brown sugar until light and fluffy.
Beat in the eggs one at a time, then mix in the vanilla extract.
Gradually add the dry ingredients to the wet mixture, mixing on low speed until combined.
Pour in the milk and mix until smooth.
Gently fold in the coarsely chopped peanut butter cups and bittersweet chocolate morsels.
Using a cookie scoop, drop tablespoon-sized portions onto baking sheets lined with parchment paper, leaving 2 inches between each cookie.
Bake for 10-12 minutes, until edges are set but centers are still soft.
Allow cookies to cool on baking sheets for a few minutes before transferring to a wire rack.
