Prepare a cookie sheet with parchment. Set to the side.
In a medium-sized bowl, mix the flour, cocoa powder, baking soda, and salt in a bowl. Set to the side.
In a medium-sized bowl with a hand mixer or in a bowl of a stand mixer with a paddle attachment, cream the butter and the sugar until light and fluffy, about 2-3 minutes. Add in the eggs and egg yolk, one at a time, followed by the vanilla.
Mix in the dry ingreidents into the butter mixture, scraping down the bowl as needed.
Mix in the milk and continue to mix until the dough is smooth.
Add in the chocolate chips and M&Ms, mixing until well distributed.
Place the cookie dough in the refrigerator and chill for 1 hour. While the cookie dough is chilling, preheat the oven to 350.
After chilling the dough, form the cookie dough balls using a medium-sized cookie scoop. Bake the cookies in the oven at 350 for 9-12 minutes.
Let the cookies chill on the cookie sheet for a few minutes before transferring them to a cooling rack. Serve while still warm for gooey, chocolate cookies.
