Preheat oven to 375ºF. Line baking sheet with parchment paper.
In a medium size mixing bowl, combine flour, baking powder, salt, and cornstarch. Set aside. 2 1/2 cups all-purpose flour, 2 teaspoons baking powder, 1 teaspoon salt, 1 tablespoon cornstarch
Using a hand mixer or stand mixer, cream together butter, brown sugar and granulated sugar until smooth. 1 cup unsalted butter, ¾ cup granulated sugar, ½ cup brown sugar
Add vanilla, the egg and the additional egg yoke and mix until well combined. ½ teaspoon vanilla extract, 1 large egg, 1 large egg yolk
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 rainbow chips and sprinkles. 1 cup rainbow chips, ½ cup sprinkles
Using a medium size cookie scoop, drop cookie dough balls onto lined cookie sheet, about 2 inches apart. Bake for 8-10 min or until the edges of the cookies are lightly browned.
Remove from oven and allow to cool for 5 min on baking sheet before transferring to a cooling rack to cool completely.
Notes
Refer to the article above for more tips and tricks.The calories shown are based on the recipe making 30 cookies, with 1 serving being 1 cookie. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. **