Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Line the bottom of a giant round cookie cake pan with a circle of parchment paper. Lightly spray the edges of the pan with baking spray, and set aside.
In the bowl of a stand mixer, beat together the butter and sugars until fluffy, about 2-3 minutes. Add the vanilla and one egg, mix, then add the egg yolk and mix until combined.
In a medium-size mixing bowl, whisk together the flour, baking soda, and salt. With the mixer on low speed, gradually add to the wet ingredients. Mix until combined.
Remove the bowl from the stand and fold in the chocolate chips and M&Ms. Pour the batter into the prepared cake pan. Smooth it out and sprinkle a few M&Ms on top. Place in the oven and bake for 16-22 minutes, or until the top turns golden brown. Remove from the oven and place the pan on a cooling wire rack. Allow to cool for 45 minutes before removing from the pan and frosting.
Frosting
Meanwhile, to prepare the frosting, in the bowl of a stand mixer, beat together the butter, vanilla, heavy cream, and salt. With the mixer on low speed, gradually add the powdered sugar. Turn the speed to high and beat until fluffy, about 5 minutes. Tint with food coloring if desired.
Pipe the frosting around the edges of the cake using a large decorating tip fitted in a piping bag. If desired, use a smaller decorating tip to pipe lettering in the center of the cake.
Notes
Adapted from Wilton's Chocolate Chip Cookie recipe.-This makes one 11-inch cookie cake, serving approximately 8 to 10 people.-You can use any 9x13 cookie bar or brownie recipe in this pan. Adjust the baking time accordingly!To STORE M&M Cookie Cake leftovers, cover the cookie cake tightly in plastic wrap or a cake container and keep it at room temperature for up to a week. For longer storage, place it in the fridge in an airtight container.Cookie Dough – Roll the dough into balls and place them on a parchment-lined cookie sheet. Freeze for about 3 hours until solid, then transfer to a freezer-safe container or bag. Freeze for up to 3 months. Bake directly from frozen, adding a few extra minutes to the bake time.Baked Cookie Cake – Allow leftover cake to cool completely before wrapping it tightly in multiple layers of plastic wrap and foil. If possible, place it in a freezer-safe bag. Freeze for up to 1 month. Let it thaw at room temperature before serving.