Preheat the oven to 350°F. Line a baking sheet with parchment paper. Set aside.
In a medium bowl, stir the all-purpose flour, baking powder, and cornstarch together. Set aside.
1½ cups all-purpose flour, 2 teaspoons baking powder, 2 teaspoons cornstarch
In a large bowl, with a hand mixer, beat the butter and sugar together on medium-high speed until smooth and fluffy. Make sure it is very light and creamed before moving on.
½ cup salted butter, 1 cup granulated sugar
Once the mixture is creamed and fluffy, add in the egg and vanilla extract. Mix together until completely combined.
1 large egg, 1 teaspoon pure vanilla extract
On low speed, add in the flour, baking powder, and cornstarch mixture. Make sure it is all mixed together by scraping down the sides of the mixer bowl.
Fold in the chocolate chips and ½ cup of the sprinkles.
¾ cup white chocolate chips, ¾ cup rainbow sprinkles
Scoop 1-inch balls of dough onto the prepared baking sheet.
Take the remaining ¼ cup of the divided sprinkles and roll each cookie in the mix, gently pressing into the sides.
Bake for 8-10 minutes. Do not let the cookies brown.
Let the cookies rest for 1-2 minutes before taking them off the sheet pan and placing on a wire rack to cool completely.