Combine melted butter, brown sugar, granulated sugar and peanut butter in a large mixing bowl and mix until well combined and smooth. Mix in milk and vanilla extract.
Combine flour, baking soda and salt in a separate bowl and whisk to blend evenly. Add this to the butter mixture and fold it in. Before all of the flour is incorporated, add the chocolate chips and mini peanut butter cups and fold them in so they are evenly distributed.
Use a 1.35-oz cookie scoop to portion dough and release the dough onto prepared baking sheets. Press a few extra peanut butter cups on top of each dough ball if desired and bake for 10-12 minutes until the surface is cracked and cookies are lightly golden. Let cookies cool on the baking sheet for 3 minutes to set before transferring individually to a wire rack to finish cooling.