Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a bowl, whisk together flour, cornstarch, baking powder, and salt.
In another bowl, beat butter and sugar until light and fluffy. Add egg yolks and vanilla, and mix well.
Gradually add the dry ingredients to the wet ingredients, mixing until combined.
Roll out the dough on a floured surface to about ¼ inch thick. Cut out circles using a cookie cutter.
Place the circles on the prepared baking sheet and bake for 10 minutes, or until the edges are lightly golden.
Let the cookies cool completely. Spread dulce de leche on the flat side of one cookie and sandwich with another cookie.
Dust with powdered sugar before serving.