Step 1

Heat the oven to 275°F. Pan-spray or line two half-sheet pans with parchment paper or silicone baking mats.

Step 2

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, sugar, and brown sugar on medium-high for 4 minutes until well combined. Scrape down the sides of the bowl, add the egg and vanilla, and mix for 4 minutes.

Step 3

Add the flour, cornstarch, baking soda, and salt and paddle on low speed until just combined, about 20 seconds.

Step 4

Add the rainbow sprinkles and mix for 10 more seconds.

Step 5

Scoop the dough into 1-tablespoon balls onto the prepared pans 2 to 3 inches apart.

Step 6

Bake at 275°F for 25 minutes. These cookies won’t change color much, so make sure you set your timer. If they don’t turn out as crispy as you would like, bake for 2 to 3 minutes longer next time.

Step 7

Let the cookies cool briefly on the pans, then transfer to a cooling rack. Share immediately unless you have enviable self-control. If you do have cookies left over, store them in an airtight container on the counter for up to 1 week.