In the bowl of an electric mixer, combine flour and sugar. Add butter and vanilla and mix until a dough forms, scraping the sides to incorporate all the flour into the dough. The dough will seem dry at this point, but keep mixing until it comes together nicely (you can add 1 Tbsp oil if you need).
Using a rolling pin, roll out the dough on a piece of parchment into a rectangle (to roughly a 9x13 size), then transfer the parchment paper to 9 x 13-inch Pyrex. Press the dough until it evenly fills the entire pan.
Add nonpareils and press with a piece of parchment so they stick to the dough.
Lift parchment out of the Pyrex. Using a pizza slicer, slice into small squares.
Transfer squares to a parchment lined baking sheet. You don’t have to place them far apart because they won’t rise significantly. Bake for 10-12 minutes. They won’t turn golden, so you can remove from oven when they’re slightly firm to the touch. They’ll continue to firm up as they cool.
Recipe by Between Carpools at https://betweencarpools.com/make-the-cutest-square-cookies-using-this-easy-hack/