Step 1

Line half sheet pans with parchment paper. Don't grease. Set aside.

Step 2

In the bowl of a stand mixer fitted with the paddle attachment (or with a handheld mixer or in a food processor), combine the butter, granulated sugar, and powdered sugar, and mix until well-combined. Add the salt and vanilla and mix.

Step 3

Add the flour (read the note below first!) and cornstarch and mix until about half of the dry ingredients are combind. Add the sprinkles and mix until evenly combined and no dry streaks remain. Try not to over mix.

Step 4

On one of the parchment-lined baking sheets, press the shortbread into a 1/4-inch (or slightly thicker) rectangle. Press additional sprinkles on top of the dough, if desired. Refrigerate the shortbread for 30 minutes (or up to 24 hours).

Step 5

Preheat the oven to 375 degrees F.

Step 6

Using a long knife or bench scraper, cut the rectangle into small squares, about 1/4- to 1/2-inch in size.

Step 7

Spread the shortbread bites out on the prepared baking sheets. They won't spread much but they shouldn't be touching. Bake for 7-9 minutes until set. For crispier shortbread, bake 1-2 minutes longer.