01 -
Preheat oven to 350°F (177°C). Line a 9-inch 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. Add egg and vanilla extract and mix well. Combine baking soda, salt, and cream of tartar, then incorporate into the mixture. Gradually blend in all-purpose flour until just combined. Fold in M&Ms and sprinkles gently.
03 -
Press batter evenly into the prepared cake pan, pressing additional M&Ms onto the top as desired. Bake for 19 to 22 minutes, until edges turn a light golden brown. Avoid overbaking as the cake continues to cook while cooling. Allow to cool completely before frosting.
04 -
Beat softened butter until smooth. Gradually add powdered sugar and beat until crumbly. Add salt and vanilla extract, then add 1 tablespoon heavy cream and beat until smooth. Add additional cream by teaspoon if needed to achieve a spreadable consistency.
05 -
Using a piping bag fitted with a 1M tip or a ziplock bag, decorate the cooled cookie cake with frosting. Garnish the top with extra sprinkles and M&Ms as desired.
06 -
Store loosely covered at room temperature for up to 2 days.