Remove the label from the sweetened condensed milk and place the can on its side in a pot of boiling water. Make sure the water covers the can completely, with at least an inch (preferably more!) of water above it.
Lower the heat and simmer for 4 hours. Make sure the water covers the top of the can at all times (you will have to pour in more water about every 30 minutes or so depending on how big your pot is).
After 4 hours, remove the can from water, allow to cool completely, then open and stir in salt. Set aside.
for the cookies
Cream the butter and the sugar together until light and fluffy. Add the egg yolks in one at a time, beating until completely incorporated. Add the vanilla extract and mix well.
Scrape down the bowl using a rubber spatula. Add the cornstarch, all purpose flour, baking powder and salt. Beat until a soft dough forms and no streaks of flour remain. Do not over mix.
Turn the dough out of the bowl and shape into two flat oval disks. Wrap tightly in plastic wrap and refrigerate for an hour until firm
Once the dough is chilled, preheat the oven to 350°F and line two cookie sheets with parchment paper. Set aside.
On a well floured surface roll out the dough into a large circe about ⅛ of an inch thick. Use a small round cookie cutter to cut out cookies, placing the shapes on the prepared cookie sheets about an inch apart.
Bake for 8 minutes or until just golden brown on the edges and dry to the touch. Allow the cookies to cool completely.
To assemble the cookies take a cookie and spready 1 teaspoon of dulce de leche over the backside. Place a second cookie on top and sandwich together. Repeat with the rest of the cookies. Enjoy!