01 -
Preheat your oven to 350 degrees. Line a 9-inch round cake pan with foil and spray with nonstick cooking spray.
02 -
Beat the softened butter and granulated sugar together with a hand mixer or stand mixer until creamy and light, about 2-3 minutes.
03 -
Beat in the egg and vanilla extract until well combined.
04 -
Mix in the baking soda, salt, and cream of tartar until combined.
05 -
Slowly mix in the all-purpose flour until the dough just comes together. Don't overmix.
06 -
Stir in the M&Ms and jimmie sprinkles until evenly distributed throughout the dough.
07 -
Press the cookie dough into the prepared 9-inch pan. Press additional M&Ms onto the top for extra color and decoration.
08 -
Bake for 19 to 22 minutes until the edges are just turning a light golden brown. It's better to underbake this than overbake - the cookie will continue cooking slightly as it cools, so err on the side of underdone for the best texture!
09 -
Let the cookie cake cool completely in the pan before frosting. This usually takes about 30 minutes to 1 hour.
10 -
Beat the softened butter until smooth. Slowly beat in the powdered sugar until the mixture is crumbly.
11 -
Add the salt and vanilla extract. Then add 1 tablespoon of heavy whipping cream and beat until smooth. Add more cream a teaspoon at a time until you reach a spreadable consistency.
12 -
Use a 1M piping tip and a piping bag or ziplock bag with the corner cut to decorate the cookie cake with frosting in fun patterns.
13 -
Top with additional sprinkles and M&Ms for a festive, fireworks-inspired look!
14 -
Slice and serve this fun, colorful dessert at your 4th of July celebration!