Preheat oven to 350°F.
Cream the butter and sugar for 3–4 minutes until light and fluffy.Beat in the eggs, one at a time.Mix in the vanilla.
In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.Add the dry ingredients to the wet ingredients and mix just until combined.Divide the dough into three equal portions.
color one portion purple .Color one portion pink.Color one portion teal
Take equal pieces from each color and gently press them together.Roll lightly into a ball, being careful not to overmix so the colors stay distinct.Place on a parchment-lined baking sheet and chill for 30 minutes .Slightly flatten the tops with your palm.
Bake for 11–13 minutes, until the edges are just set and the centers still look soft.As soon as the cookies come out of the oven, immediately sprinkle with your favorite sprinkles while they're still warm so they stick perfectly. Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack.