In a small bowl, whisk together the flour, cornstarch and salt. Set aside. Then, in a seperate bowl, use a hand mixer to cream together the butter and confectioners sugar for 2 minutes. Then beat in the vanilla. Mix the dry ingredients into the wet, adding it in batches until it is all combined. The dough will be sticky.
Cover the dough tightly and refrigerate for a mininum of 30 minutes. Once dough has chilled long enough and you're ready to bake, preheat the oven to 350F. Line 2-3 baking sheets with parchment paper or a silicone baking mat and set them aside.
Roll the dough into small balls (no bigger than 1 tbsp of dough per ball). Try to also roll them quickly, as the dough will grow extremely sticky again once warmed from your hands. Place the dough balls onto the prepared baking sheets, with an half an inch or so of space between them (they should not spread all that much). Slightly flatten the balls with your fingers and gently press sprinkles into the flattened balls.
Bake for 8-10 minutes, until cookies have browned very slightly around the edges. Cool on the baking sheets for 2 minutes, then transfer cookies onto a wire rack to cool completely. I recommend using a spatula to move your cookies from the baking sheet, as they are very delicate and crumble easily. Store them in an airtight container for up to 1 week.
