Preheat your oven to 325°F (about 160°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal.
In a small bowl, whisk together the all-purpose flour and cornstarch until well combined. Set aside.
In the bowl of an electric mixer, combine the cold unsalted butter, powdered sugar, and vanilla extract. Beat on medium speed until the mixture is smooth and creamy, scraping down the sides of the bowl as needed.
Reduce the mixer speed to low. Gradually add the flour–cornstarch mixture, mixing just until the dough starts to come together and it looks like coarse crumbs.
If using sprinkles, add them now and mix briefly until evenly distributed.
Using clean hands, gently press and knead the dough in the bowl or on a clean surface until it forms a smooth, cohesive ball.
Transfer the dough to the prepared 9×13-inch pan. Press it evenly into the bottom of the pan, making sure the surface is as flat and compact as possible.
Place the pan in the refrigerator and chill the dough for 1 hour.
After chilling, use the parchment paper to lift the slab of dough out of the pan and onto a cutting board. Cut into small, bite-sized squares.
Arrange the squares about 1 inch apart on an ungreased baking sheet.
Bake for 10–12 minutes, or until the bottoms of the bites are just starting to lightly brown while the tops remain pale. Watch closely near the end, as they can brown quickly.
Remove from the oven and let the bites cool on the baking sheet for a few minutes, then transfer to a wire rack to cool completely. Enjoy!