In a medium mixing bowl, whisk together the all-purpose flour, baking powder, cornstarch, and salt. Set aside.
2 cups all-purpose flour, 1½ teaspoons baking powder, 1 teaspoon cornstarch, ½ teaspoon salt
Use a stand mixer, or a mixing bowl with an electric mixer, and cream together the butter and sugar until light in color and fluffy (about 2-3 minutes). Scrape down the sides of the bowl with a spatula.
1 cup (2 sticks) unsalted butter, 1 cup granulated sugar
Beat in the egg and vanilla extract and mix until combine.
1 large egg, 2 teaspoons vanilla extract
Add the dry ingredients into the wet ingredients and mix on low speed (increase speed as needed) until the dough comes together. It will be sticky and seem wet, but that's ok.
Create a ball of dough and place it on a sheet of plastic wrap and wrap it up tightly. Place the dough in the fridge to chill for 1 hour.
While the dough is chilling, preheat the oven to 350℉. Prepare two cookie sheets (12"x17") and line with parchment paper or a silicone baking mat. Add the nonpareil sprinkles into a small bowl.
1 cup red & green nonpareils
Once the dough has chilled, use a 2 tablespoon cookie scoop to scoop the dough and roll it into a ball. Roll the dough ball in the sprinkles until it's well coated. Place 12 cookies per cookie sheet.
Bake for 15-20 minutes or until the edges are slightly golden brown (still pretty pale) and the cookies are set.
Let the cookies cool on the warm cookie sheet for 5 minutes before transferring them to a cooling rack to cool completely. Or eat one warm!