In the bowl of a stand mixer beat butter and sugar on medium speed until light and fluffy, about 5 minutes. Add in vanilla and the egg and mix until just combined.
While the mixer is on low, add in the corn starch, baking soda, baking powder, salt, and flour until everything is incorporated. Add in the sprinkles and mix on low until just combined.
Line a baking sheet with parchment paper. Using a medium sized cookie dough scoop (about 2 ½ tablespoons of dough per cookie) scoop onto a baking sheet lined with parchment paper. If the dough seems soft, cover the pan with plastic wrap and chill the dough balls for 30 minutes or up until 24 hours. This will help to prevent the cookies from spreading and add flavor!
Toward the end of the chilling process preheat the oven to 350 degrees. Line 2 baking sheets with parchment paper and arrange 4-5 dough balls about 2 inches apart. Let them sit at room temperature for 5 minutes before baking.
Bake for 10-12 minutes, or until the edges of the cookie are just set. They may seem underdone but they will be perfect! Remove from the oven and if you want perfect circular cookies use a round cookie cutter larger than the cookie to scoot them into a round shape. Let them cool for 5 minutes on the cookie sheet before transferring to a cookie rack to cool completely.