FOR THE COOKIES: In a medium size bowl whisk the flour with the salt. In a larger bowl, beat the butter until smooth and creamy. Add the sugar and beat until smooth. Beat in the Vanilla Extract.
Gently beat in the flour mixture just until incorporated. Flatten the dough into a disk shape, wrap in plastic wrap, and chill the dough for at least an hour, or until firm.
Preheat oven to 350 degrees F -with the rack in the middle of the oven. Line two baking sheets with parchment paper.
On a lightly flourd srface, roll out the dough until it is about 1/4 inch thick. Using a lightly floured 1-1/2 or 2 inch round cookie cutter, cut dough. Place on baking sheets and put in refrigerator for about 15 minutes -this firms the dough and helps cookies keep shape while baking.
Bake in preheated oven for 7-10 minutes -until edges of cookies are light brown around the edges. Cool on a cooling rack. Makes 24-36 unfilled cookies.
FOR THE FILLING: Empty the contents of a can of Sweetened Condensed Milk into an oven-proof dish; sprinkle with some kosher salt and tightly cover it with foil.
Place the covered dish in a larger roasting or casserole pan and fill it up with water until it reaches three quarters of the up the covered dish to create a water bath. Bake at 425 degrees F for 2-1/2 to 3 hours, checking every 30 minutes on the water level and adding more as needed.
NOTE: Dulce de Leche is ready when it takes on a brown and caramel-like appearance. Remove from the oven and whisk to smoothness.
Let cool before filling and/or storing. This filling will keep in the refrigerator at a least a week. Yummy!
To make sandwich cookies, spread the cooled Dulce de Leche filling on the flat side of a cookie (Don't spread it thick or it will ooze out when you put on the top). Then top with another cookie -flat side on top of filling. Sprinkle cookies with Confectioners sugar.
FOR ANOTHER TREAT: You can make a Butter Cream Frosting for a yellow or caramel cake using the Dulce de Leche filling, by adding powdered sugar, butter and cream.