Alfajores, these crisp wafer cookies, are filled with caramelized sweetened condensed milk. Although you often can find a full-fat version of this product at Hispanic markets, we've made our own lower-fat version.
Sift flour and 3/4 cup sugar together twice. Place flour mixture, butter, and egg yolk in a large bowl; beat with a mixer at low speed 3 minutes (dough will be crumbly). Turn dough out, and knead until combined. Divide dough in half; gently press each portion into a 4-inch circle on plastic wrap. Cover and chill 30 minutes or until firm.
Preheat oven to 350°.
Working with 1 dough portion at a time, unwrap dough and place on 2 sheets of overlapping plastic wrap. Cover dough with 2 additional sheets of overlapping plastic wrap. Roll dough, still covered, into a 10-inch circle. Freeze dough 5 minutes or until plastic wrap can be easily removed. Remove plastic wrap; cut dough into 15 circles with a 2-inch biscuit cutter. Repeat procedure with remaining dough. Place dough circles 1 inch apart on baking sheets lined with parchment paper. Bake at 350° for 7 minutes (should not brown). Cool on a wire rack.
Gently spread 1 teaspoon of Caramelized Condensed Milk onto each of 15 cookies, and reserve the remaining caramel for another use. Gently top with remaining cookies. Sprinkle the cookies with 3 tablespoons powdered sugar.