Preheat oven to 350°F. Spray a heart-shaped baking pan with baking spray or line with parchment paper. Set aside.
In a large bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.
In the bowl of a stand mixer, beat together the softened butter and both sugars until fluffy, about 3 minutes. Scrape down the sides every minute or so to ensure all of the butter and sugars are well incorporated.
Beat in the egg and vanilla.
Gradually add the dry ingredient mixture to the wet ingredient mixture, scraping down the sides as needed to incorporate all of the ingredients. Mix together just until all ingredients are well incorporated.
Using a silicone spatula or wooden spoon, gently fold in most of the semi-sweet chocolate chips, white chocolate chips, and M & Ms; save some for the next step.
Gently press the batter into the baking pan, filling in the whole shape and trying to press it into an even layer.
Gently press the white sprinkles and the remaining chocolate chips and M & Ms onto the top of the batter.
Bake at 350°F for 25 - 35 minutes, just until a toothpick inserted into the center of the cookie cake comes out clean. The edges will be golden brown.
While the cookie cake is baking, make the vanilla buttercream frosting. Color with pink food coloring if desired. Transfer to a piping bag.
After the cake is done baking, allow to cool in the pan for 10 minutes before inverting and transferring to a plate. Allow the cookie cake to cook completely before decorating with the vanilla buttercream frosting.
Notes
Makes approximately 10-12 servings.Frosting: You can use storebought frosting, or use our Easy Vanilla Buttercream Frosting recipe.Storage: Cover with plastic wrap or aluminum foil and store at room temperature for up to 4 or 5 days. You can also store slices in an airtight container at room temperature for up to 3 or 4 days, or in the refrigerator for up to one week.