Preheat the oven to 350 degrees. Add parchment paper to a baking sheet.
Mix the cake mix, melted butter, cake mix, eggs, and cornstarch into a bowl. Use a hand mixer to combine—place in the fridge to firm up the dough.
Use a cookie scoop (small) and add 12 cookie dough balls to the baking sheet. Put in the oven and bake for 8-11 minutes or until the bottoms of the cookies turn golden brown.
Once the cookies are done baking, please remove them and place them on a wire rack to cool.
As the cookies are cooking, it's time to make the frosting.
Instructions for Frosting:
Place the softened butter into a mixing bowl and beat until fluffy. This will take about 2 minutes.
Mix in the vanilla, milk, and powdered sugar. Mix on medium until small peaks form. If you are adding food coloring, do it now.
Put the frosting into a prepared bag and frost each cookie. Add sprinkles. (You can also just add the frosting with a knife)