Cream butter and peanut butter together for 3 minutes in the bowl of a stand mixer on medium speed. Add sugars with the mixer running on low. Turn speed up to medium high and beat for 2-3 minutes until fluffy and light in color. Add eggs one at a time, incorporating the first egg fully before adding the second one. Add vanilla and milk and mix on low until incorporated. Add flour mixture and mix on low until just incorporated. Don’t overmix the batter, you don’t want dense and tough cookies.
Add M&M’s and chocolate chips and mix on the stir setting or fold them in until just distributed. Scoop 1 heaping tablespoon sized dollops onto parchment paper or silicone mat lined baking sheet. Roll into a ball and slightly press the tops down until they are flat on top but not smushed. Chill in the refrigerator for at least 1 hour. (At this point, you can chill the cookies in the freezer and place them into a Ziploc bag once frozen and have cookies at hand to bake and eat whenever it pleases you or whenever you have company over.)
Preheat the oven to 375F. (Here you can add additional M&Ms and chocolate chips to the top). Bake cookies for 8-10 minutes until slightly brown. Don’t overbake them, there is nothing sadder in life than an over baked cookie!
Right before baking, place a couple M&M's on top of each cookie to make them look prettier.