Line a baking sheet with parchment or foil.

In a food processor, pulse together flour and confectioners sugar. Add the butter and pulse until the mixture resembles coarse meal, about 20 seconds. With machine running, pour in the water in a slow steam and process just until the dough comes together, about 20 seconds. Form the dough into 2 flattened disks and wrap well in plastic. Refrigerate 1 hour.

Preheat oven to 350 degrees. Flour your counter or work service and roll out one of the disks of dough to a about ¼-inch thickness. Using a 1 ¾-inch round cookie cutter, cut out rounds from dough and transfer to prepared baking sheet. Do the exact same thing with the other disk of dough. Gather up the scraps from both batches, and reroll and cut. Sprinkle half the rounds with sanding sugar - these will be the top of the cookie sandwich. Bake until golden brown, about 15 minutes, rotating sheets halfway through. Transfer to a wire rack to cool completely.

Right before serving, spread 1 or 2 teaspoons of the dulce de leche on bottom of unsugared cookies. Place sugared cookies on top to make sandwiches. Unfilled cookies can be stored in an airtight container at room temperature up to 3 days. Remaining dulce de leche can be stored, covered, in the refrigerator as well.