Step 1

Preheat oven to 350°F. Line a 9” round cake pan with foil and spray with nonstick cooking spray.

Step 2

Make the cookie cake: Beat butter and sugar with a hand or a stand mixer until creamy. Beat in egg and vanilla, then mix in baking soda, salt, and cream of tartar. Slowly mix in flour. Stir in M&Ms and sprinkles.

Step 3

Press batter into prepared pan, pressing more M&Ms onto the top as desired. Bake for 19-22 minutes or until the edges are just turning a light golden brown. It’s better to under bake this dough than over bake it. The cake will continue cooking a bit as it cools so err on the side of underdone for the best tasting cookie. Let it cool completely before frosting.

Step 4

Make the frosting: beat butter until smooth, then slowly beat in powdered sugar until crumbly. Add salt and vanilla, then add 1 tablespoon of heavy whipping cream. Beat until smooth, adding more a teaspoon at a time to get a spreadable texture.

Step 5

To frost: use a 1M tip and a ziplock or piping bag to decorate the cookie cake. Top with more sprinkles and M&Ms.

Step 6

Store loosely covered for up to 2 days.