Preheat oven to 375°F (190°C). Line baking sheet with parchment paper.
In a medium size mixing bowl, whisk together flour, baking powder, salt, and cornstarch. Set aside.
Using a hand mixer or stand mixer, cream together butter, brown sugar and granulated sugar until smooth.
Add vanilla, the egg and the additional egg yolk and mix until well combined.
Gradually add in dry ingredients a little at a time, and mix on a low speed until combined, then increase to medium speed until all ingredients are blended.
Using a wooden spoon or a rubber spatula stir in sprinkles and rainbow chips.
Using a medium size cookie scoop, drop dough onto lined baking sheet, about 2 inches apart.
Bake for 8-10 minutes or until the edges of the cookies are lightly browned and tops are puffy but set.
Remove from oven and allow to cool for 5 min on baking sheet before transferring to a cooling rack to cool completely.