Preheat oven to 350°. Line large baking sheet with parchment paper. In a food processor, pulse the butter with 1/2 c. of the powder3ed sugar until combined. Add the cornstarch, vanilla and flour and process until a soft dough forms. Transfer the dough to a work surface and divide it into 4 equal pieces.

On a lightly floured work surface, roll each piece of dough into a 12" rope. Cut each rope into 12 pieces and roll each into a ball. Transfer the balls to baking sheet. Bake the cookies on the center rack of the oven for about 22 minutes.

Sift the remaining 1 cup of sugar into a bowl. Add 12 warm cookies to the bowl at a time and toss to coat. Transfer the cookies to a clean baking sheet. While the cookies are still warm, poke 4 shallow holes into each one of the cookies with a toothpick to make a buttonhole pattern. Let cool completely.  Makes 4 dozen.