Fill a large pot with enough water to cover the can of sweetened condensed milk.

Bring to a boil over high heat, then reduce to a simmer.

Simmer for 2 1/2 to 3 hours, ensuring the can is always covered with water.

Remove the can from the water and let it cool to room temperature.

Preheat oven to 350F.

In a medium bowl, combine flour and salt.

Set aside the flour mixture.

In the bowl of a stand-up mixer, cream together butter, vanilla, and sugar for 3-4 minutes, until light and fluffy.

Gradually add the flour mixture to the butter mixture, mixing until just combined.

Shape the dough into a disc, wrap in plastic, and refrigerate for 30 minutes.

On a heavily floured surface, roll out the dough to 1/2-inch thickness.

Use a 1 1/2-inch round cutter to cut out shortbread cookies.

Place cookies on a Silpat or parchment paper lined baking sheet.

Poke a series of three holes, using a fork, in the center of each cookie.

Transfer cookies to the oven and bake for 12-15 minutes, or until edges are slightly golden brown.

Remove pan from the oven and let cookies cool on a wire rack.

Open the can of dulce de leche.

Lay half of the shortbread cookies bottom-side up.

Spread about 1/2 teaspoon of dulce de leche on the upside down cookies.

Top with the other half of the cookies and lightly press down until you've gone through all the cookies.