Preheat oven to 375 degrees. Grease a mini muffin tin and set aside.
For the cookies, whisk together the flour, baking soda and salt and set aside.
In a large bowl with a stand mixer or electric mixer, beat the butter and peanut butter together until well combined. Add the sugars and beat until fluffy, about 2 minutes. Add the egg, milk and vanilla and beat until combined.
Shape the dough in one inch balls and place one in each muffin cup. Flatten slightly to shape to the muffin tin. Bake for 8-10 minutes until slightly golden and edges turn golden brown.
When they're done, using the bottom of a small round object, like a small spice bottle or the end of a rolling pin, push the middle of the cookie in to create a cup form. Allow to cool in pan for 3 minutes and transfer to wire rack to cool completely.
In the meantime, make the cookie monster dip by whipping the cream cheese, butter and peanut butter together using an electric mixer until smooth. Mix in the brown sugar, flour, vanilla and oats.
Beat in the powdered sugar 1/2 cup at a time. Add the M&Ms and mini chocolate chips and stir tother using a rubber spatula.
Use a small cookie scoop or 1/2 tablespoon to fill cookie cup with cookie monster filling. Serve immediately or keep in an air tight container in the refrigerator for up to a week.