Alfajores

Preheat the oven to 400 degrees F.
Pour the sweetened condensed milk into a small pyrex baking dish and cover it with foil. Place the smaller baking dish into a larger pyrex and fill the sides with hot water. (a little more than half way) This is your water bath for the sweetened condensed milk to cook in. Bake about 1 hour and 15 minutes. It will be a nice caramel color.
Remove it from the oven and let it rest 15 minutes. Remove foil and pan from the bath. Whisk smooth and cool.

In a bowl, whisk together the all-purpose flour, sifted corn starch, powdered sugar and salt. Set aside.
Beat butter and vanilla in a mixer. Add flour mixture slowly and combine.
Form it into a ball on plastic wrap and press it into about a 2-inch round. Wrap it tightly in plastic wrap and transfer to the fridge to chill for about 30 mins.
Preheat the oven to 350 degrees F.
Transfer the dough to the center of a sheet of parchment. Place a sheet of parchment on top and roll it out slowly. Roll it out 1/2 to 1/4-inch thick.
Using a 2-inch cookie cutter, cut out cookies. Transfer to parchment-lined baking sheet and bake 11-13 mins. Cool.

When the cookies have cooled, flip half over and spread on the caramel filling. Top with other cookie. Dust with powdered sugar.