01 -
Preheat oven to 350°F (175°C). Line a 9” (23 cm) round cake pan with foil and spray with nonstick cooking spray.
02 -
Beat softened butter and granulated sugar with a hand or stand mixer until creamy. Mix in the egg and vanilla extract. Add baking soda, salt, and cream of tartar, and mix until combined. Gradually add the all-purpose flour. Incorporate M&Ms and sprinkles into the dough.
03 -
Press the batter evenly into the prepared cake pan. Optionally, press additional M&Ms onto the surface. Bake for 19-22 minutes, or until the edges have a light golden brown color. Avoid overbaking—slightly underdone results in the best texture. Allow the cake to cool completely before adding frosting.
04 -
Beat softened butter until smooth. Slowly incorporate the powdered sugar until the mixture becomes crumbly. Add salt, vanilla extract, and 1 tablespoon of heavy whipping cream, and beat until the texture is smooth. Add additional cream, a teaspoon at a time, to reach a spreadable consistency.
05 -
Using a 1M piping tip and ziplock or piping bag, decorate the cooled cookie cake with the frosting. Garnish with additional M&Ms and sprinkles.
06 -
Store the cake loosely covered at room temperature for up to 2 days.