How to Make It
In a medium bowl, whisk the flour with the baking powder, salt and cornstarch. In the bowl of a stand mixer fitted with the paddle, cream the butter with the sugar at medium high speed until light and fluffy, about 4 minutes. Add the egg, lemon zest and vanilla and beat at medium speed until fully combined, scraping down the bowl as needed. At low speed, add the dry ingredients and beat until incorporated. Beat in the sprinkles until evenly distributed.
Using a 1-ounce ice cream scoop or 2 tablespoons, scoop 1-inch balls of dough onto a baking sheet lined with parchment paper. Refrigerate for 1 hour.
Preheat the oven to 325°. Arrange 12 balls of dough about 2 inches apart on each of two baking sheets lined with parchment paper. Using the flat bottom of a glass, flatten the balls to form cookies that are 2 inches wide and 1/2 inch thick.
Bake the cookies for 16 to 18 minutes, until light golden brown; shift the pans from top to bottom and front to back halfway through baking. Let the cookies cool on the sheets for 5 minutes, then transfer to racks to cool completely. Repeat with the remaining dough.