Preheat your oven to 350 degrees. Line whatever pan you are using with parchment paper or spray with cooking spray throughly.
In a large bowl, sift together the flour, cornstarch, baking powder, and salt and set aside.
In the bowl of an electric mixer, cream both sugars, butter, and vanilla extract on medium speed until light and fluffy, about 3 minutes total.
Add the egg and the yolk and mix well. Slowly add the dry ingredients into the wet ingredients at low speed. Add in the chocolate chips and the sprinkles.
Press cookie dough into prepared pan and try to make it as even as possible. Bake anywhere from 30-45 minutes, or until the edges turn a nice golden brown color.
As I mentioned above, baking time will vary based on what pan you use. Mine took a total of about 42 minutes. Remove from oven and cool completely on wire rack. When cool, remove cookie cake from pan and place on cake board or large platter.
To make buttercream, beat the butter in the bowl of a stand mixer for about 3 minutes.
Then, I added in half the powdered sugar and continue to beat until evaporated into the butter.
Add in the remaining powdered sugar, salt, and vanilla.
hen, add one tablespoon of milk or cream in at a time, until mixture is light and fluffy. I used 2 tablespoons of heavy cream.
Also, if you are using the red food coloring, add it in now until you achieve a light pretty pink color, or a darker red color, whatever you prefer!
To decorate, add buttercream into piping bag and decorate cake to your heart's content!
To store leftover cake, you could go ahead slice it all and place in an airtight container. Or cover the entire cake with plastic wrap, which is what I did. Cake will keep for about 3 days at room temperature.
