Pour the sweetened condensed milk into a 9-inch deep dish ceramic pie pan.
Cover tightly with aluminum foil and set into a roasting pan.
Bring a kettle of water to boil and pour the water into the roasting pan so that it comes halfway up the pie plate.
Bake until the milk has turned a caramel color, 1 ½ to 2 hours (check the water level after about 45 minutes and refill the hot water if necessary).
Let cool, then strain into a glass bowl.
For the cookies:
Whisk flour, salt and spice together in a bowl. Set aside.
Whip butter and sugar until well combined and creamy.
Whip in the vanilla extract.
Whip in the dry ingredients and keep whipping until the dough starts to clump together.
Bring the dough together into a ball by hand, cut the ball in half and form each half into a disk.
Wrap each disk in plastic wrap and chill in the fridge for 30 minutes.**
Heat the oven to 350 degrees F.
Take the disks out of the fridge, roll them out between two sheets of plastic wrap, then cut out the cookies.***
Bake the cookies on cookie sheets lined with parchment paper until they are lightly browned (12 – 15 minutes).
Let the cookies cool completely.
Sift powdered sugar onto the top cookies.
Spoon a dollop of dulce de leche onto the bottom cookies (about 1 teaspoon per cookie) and top with a cut-out center cookie.
*Alternatively you can use store-bought dulce de leche (available in a can) instead of making your own.**You can make the dough up to one week ahead and keep in the fridge. When you're ready to bake the cookies, let the dough warm up at room temperature until it's easy to roll (30 to 40 minutes).***Using plastic wrap allows you to roll out the dough without adding any additional flour, which would make the dough brittle. Here is a tutorial on how to use a Linzer cookie cutter.
Spiced Dulce de Leche Cookies https://thespicetrain.com/spiced-linzer-cookies-with-dulce-de-leche/