FOR THE COOKIES:

1. PREP: Preheat the oven to 350 degrees. Line a cookie sheet or two with a silicone baking mat or parchment paper. Set aside.
2. CREAM BUTTER & SUGAR: In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar together until combined.
3. ADD VANILLA: Add the vanilla extract. Stir to combine.
4. SHIFT FLOUR & SALT: Sift together the flour and salt in a medium bowl.
5. MIX INGREDIENTS: Next, add the flour mixture to the butter mixture. Mix on low speed until the dough starts to come together.
6. FORM DOUGH & CHILL: Dump the dough onto a surface dusted with flour. Shape the dough into a flat disk, wrap it in plastic, and then chill for at least 30 minutes. You can also divide the dough into two discs and freeze one for later use.
7. ROLL OUT & CUT COOKIES: On a lightly floured surface, use a floured rolling pin to roll out the disc of cookie dough until it’s 1/4-inch thick. Cut dough into round cookies using a small (2″) cookie cutter. If the dough cracks, just wait a few minutes for the dough to warm up. Reroll the remaining dough and continue cutting until all of the dough is used. 
8. BAKE: Place the cookies on a baking sheet about 2 inches apart. Bake until the edges of the cookies are just beginning to turn golden, about 10 minutes.
9. COOL: Allow cookies to cool on the baking sheets for 5 minutes. Then transfer all of the cookies to a wire rack to cool completely before assembling.
10. FILL: Spread approximately 1/2 teaspoon of dulce de leche on each bottom cookie. Then carefully top each with a top cookie and press down gently to create a cookie sandwich. 
11. DUST: Dust the tops of the cookies in a sprinkling of powdered sugar. You can also roll them in shredded coconut if you prefer (the coconut will stick to the dulce de leche).
12. STORE: Store the cookies in an air-tight container at room temperature for 2 days or in the refrigerator for up to 1 week.

FOR THE DULCE DE LECHE: 

• ADD CAN OF CONDENSED MILK TO POT OF WATER: Submerge a sealed can of sweetened condensed milk in a large pot filled with water. The water should cover the can by about 2 inches. Since dulce de leche will last in the refrigerator in a sealed can for up to 3 months, you might want to make more than one can at a time.
• SIMMER: Bring the pot to a boil, then cover and reduce to a simmer. Simmer the can(s) for 3-3 1/2 hours.
• CHECK WATER LEVEL REGULARLY: Make sure to check the pot’s water level at least hourly. Add more water if needed. The can(s) needs to stay submerged in water to prevent them from bursting.
• COOL: Remove the can(s) with tongs and set it on a wire cooling rack. Let cool to room temperature before opening.
• STORE: You can store dulce de lech in an air-tight container in the fridge for up to two weeks and up to 3 months in an unopened can.