In the bowl of a stand mixer with a paddle attachment or an electric mixer, beat on medium-high speed; cream the butter, 1/4 cup sugar, and brown sugar until light and fluffy (2 minutes)
Add the egg and vanilla and mix until well combined (about 1 minute)
In a medium bowl, whisk flour, baking powder, cornstarch, and salt.
Add the flour mixture to the wet ingredients and mix on low speed until the dough is just combined and comes away from the side of the bowl, scraping down the sides of the bowl.
Cover the bowl with plastic wrap and chill the sugar cookie dough in the refrigerator for 1-2 hours (the longer, the better, as the flavor will absorb more).
Scoop the cookie dough and shape it into 2-3 inch balls, depending on how big you want your cookies.
Using your thumb, make a hole about 3/4 of the way into the center of the cookie dough ball. Place 1-2 tsp of chocolate chips, depending on how large your cookie balls are, and press the sides of the dough ball together to seal in the chocolate chips.
Place onto prepared baking sheets lined with parchment paper lined baking sheet a silicone mat.
Add the sprinkles to the top of each cookie and Bake in the oven at 375 degrees for 12-14 minutes. Remove when the center still seems slightly underdone. They will continue baking on the cookie sheet, and you want the center to be chewy when they are done.