In the bowl of a stand mixer, beat together the butter and powdered sugar until light and fluffy.
Mix in the flour and cornstarch until well combined.
For thick shortbread bars, pat dough into a 9x13 baking dish. Prick with a fork. For thinner shortbread bars, pat into an ungreased 10x15 rimmed baking sheet. Prick all over with a fork.
Sprinkle the bars with sprinkles.
Bake for 50 minutes (40 minutes for thinner bars) or until the edges are lightly golden and the cookie is set.
Cut into squares while warm, but do not remove from pan until fully cooled.