How to Make It
Preheat the oven to 350°. Line a large baking sheet with parchment paper. In a food processor, pulse the butter with 1/2 cup of the confectioners' 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-inch rope. Cut each rope into 12 pieces and roll each piece into a ball. Transfer the balls to the baking sheet; they won't spread much. Bake the cookies on the center rack of the oven for about 22 minutes, until the bottoms are golden but the tops are still pale.
Sift the remaining 1 cup of confectioners' 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 with a toothpick or skewer to make a buttonhole pattern. Let the cookies cool completely before serving or storing.