Sift the cornstarch, flour and baking powder together in a medium bowl. In a mixer bowl, beat the butter and sugar together until light and fluffy. Add the egg yolks, beating until they are incorporated.
2
Reduce the mixer speed to low and add the flour mixture in 3 batches, allowing each batch to become incorporated before adding the next.
3
Form the dough into a disk without handling it too much. Wrap with plastic wrap and refrigerate for at least 1 hour.
4
Preheat oven to 350̊F. Remove the dough to a lightly floured work surface an roll out to about 1/8-inch thick. Cut out 40 2-inch rounds, and carefully place the rounds on two lightly greased cookie sheets.
5
Bake for 9 to 10 minutes, but not so long that they begin to brown. Remove from the oven and cool for 5 minutes. Then remove the cookies to wire racks and cool completely.
6
Spread about 2 teaspoons of dulce de leche on the flat half of a cookie and cover it with the flat half of another cookie to form a sandwich, Repeat with the remaining cookies.
