Butter Cookies with Dulce de Leche

Put oven rack in middle position and preheat oven to 350 °F. Butter a large baking sheet. Whisk together cornstarch, 3/4 cup flour, baking powder, and salt in a small bowl. Beat together butter and sugar in a large bowl with an electric mixer until light and fluffy, then beat in egg yolks, Pisco, and vanilla. Stir in flour mixture until combined, adding 1 to 2 tablespoons flour if dough is sticky. (Dough should be soft.) Form dough into a disk and roll out into an 11-inch round (1/8 inch thick) on a lightly floured surface with a lightly floured rolling pin. Cut out 32 rounds with cutter (reroll scraps if necessary) and transfer to sheet, arranging rounds 1/2 inch apart. Bake until firm and pale golden around edges, 12 to 15 minutes. Transfer to a rack and cool 10 minutes. Sandwich cookies with about 1/2 teaspoon dulce de leche. Dust with confectioners sugar.