Pour 1 can of sweetened condensed milk into the to of the double boiler and add 1 cinnamon stick

Place over boiling water and cook over low heat, stirring occasionally until thick and caramel colored (approximately 2-3 hours)

Remove cinnamon stick

Remove from heat and allow to cool

Preheat oven to 350 degrees F

Prepare baking sheets with butter or cooking spray

Combine flour and salt in a bowl and set aside

Beat butter and sugar until pale and creamy

Add vanilla extract

Add flour mixture and combine until a soft dough forms

Shape dough into a ball and refrigerate for 30 minutes

Generously flour work surface and rolling pin and roll out dough to 1/8 inch thickness

Use a 2-inch round or fluted round cookie cutter to cut out 12 cookies

Use a fork to prick center of each cookie to prevent bubbling during the baking process

Bake at 350 degrees F for 10 minutes (do not allow cookies to brown)

Remove from baking sheet and cool completely on wire rack

Spread 1-2 teaspoons of dulce de leche onto the flat side of 6 cookies. Top each with the remaining 6 cookies

Sprinkle the tops of each sandwich cookie with powdered sugar, if desired

Serve immediately