In a large bowl, use an electric mixer to combine the butter and powdered sugar until light and fluffy- for a FULL 5 minutes. Gradually add in the flour and cornstarch, beating until well blended- about 3 more minutes. Beat in the vanilla.
With hands lightly dusted with additional cornstarch, roll the dough into 1-inch balls. Place 1-inch apart on baking sheets that are lined with parchment paper or silpat mats. Press lightly with a powdered sugar fork. Sprinkle nonpareils on top of each cookie.
Bake at 300℉ until the bottoms of the cookies are lightly browned, 20 to 22 minutes. Cool for 5 minutes before removing the cookies from the pans to wire racks to cool completely.
Notes
If you want to bake a LOT of these for a party or a cookie exchange, use the following amounts to make 100 (small) cookies: 3 cups butter, 2 cups powdered sugar, 4½ cups flour, 1 cup cornstarch and 2 teaspoons vanilla.