Preheat the oven to 350ºF

Line baking sheets with parchment paper.

In a large mixing bowl, combine the dry ingredients by sifting together the all-purpose flour, cornstarch, baking powder, and salt. Set aside until ready to combine with the wet ingredients.

In a separate bowl, beat the softened butter until creamy.

Add in the granulated sugar until the mixture is fluffy.

Beat in the egg and egg yolk as well as the vanilla extract until mixed well.

Stir in the sprinkles and white chocolate chips. Use a spoon to prevent the sprinkles from bleeding into the dough.

Scoop the cookies using a #40 cookie scoop or by scooping the dough by 1 and a half tablespoons of dough. Bake one cookie sheet at a time.

Bake for 10-12 minutes, or until the edges are lightly golden brown.

Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.

Store in an airtight container.