Preheat oven to 350°F. Spray a 9 inch springform pan with baking spray. Set aside.
In a medium bowl, mix flour, baking powder, baking soda, corn starch, and salt. Set aside.
With a hand mixer or with a stand mixer fitted with a paddle attachment, cream butter and sugar until smooth and creamy, about 2 minutes.
Add whole egg and egg yolk one at a time. Add vanilla extract.
Slowly add the dry ingredients to the wet until combined in the bowl of your stand mixer.
With a rubber spatula, fold in sprinkles and white chocolate chips.
Spread cookie dough onto 9 inch pan. Bake in oven for 25-30 minutes, or until golden brown.
Cool on a wire rack.
With a hand mixer or stand mixer fitted with a paddle attachment, cream butter until smooth.
Add 1 cup of confectioner's sugar, then add 1 tbsp of half and half. Alternate adding sugar and half and half until desired consistency. (I used a total of 3 cups of confectioner's sugar and 2 tbsp of half and half.)