Preheat oven to 350°F.
In a medium bowl mix the flour, corn starch and confectioners' sugar.
Add the diced butter and mix well until with your hand, pressing the butter pieces into the dry ingredients.
At first the mix will be dry and crumbly but keep working the dough until it's soft and uniform, about 10 minutes.
Roll the dough about 1 inch thick.
Cut the cookies with a biscuit cutter and place in a baking sheet lined with parchment paper.
Bake at 350°F for 8 minutes. Let the cookies sit in the baking sheet for about 5 minutes.
Let the cookies cool down completely before adding the Dulce de Leche.
Once cooled down, add a layer of dulce de leche. I like to use a pipping bag for this to avoid pressing too hard on the cookie.
Add the other cookie to form the sandwich and sprinkle with confectioner's sugar (I like to add a rack on top of a baking sheet to dust them with sugar).