Preheat oven to 300° degrees F. Line 2 baking sheets with parchment paper or silicone liners. Set aside.
In the bowl of a stand mixer, cream together the butter and confectioners' sugar with the paddle attachment on medium-high speed for 5 minutes, until soft and very fluffy. Stop to scrape down the sides of the bowl as needed. Don't skimp on the time - it should whip for 5 minutes!
Add in the vanilla extract and mix well, then add in the cornstarch and flour and mix on low speed until just combined.
Using a Tablespoon-size cookie dough scoop, scoop out dough balls and roll in your palms to smooth them. Place 1-2" apart on the baking sheets. Take a fork and gently press into each dough ball to slightly flatten. You may want to dip the fork in some flour if it sticks. Immediately sprinkle with the non-pareils, if using.
Bake for 20-22 minutes, switching and rotating pans halfway through baking time to ensure even cooking. (I take the baking sheet on the top rack and put it on the bottom rack halfway through, and vice versa). Cool on the baking sheets for about 15 minutes, then transfer to wire racks to cool completely.