This traditional cookie, probably native to Spain, is widely enjoyed not only in its native country but in southern France, the Philippines, and in many South American countries. Classically, two shatteringly crisp nut shortbread wafers sandwich velvety dulce de leche for a luxurious and decadent sweet; the recipe here reflects that tradition.
To toast the nuts: Preheat the oven to 300°F.
Place the nuts in a single layer on a baking sheet and toast until light golden brown, about 14 minutes. Remove from the oven and cool completely.
When cool, place the toasted nuts in the bowl of a food processor with 1/2 cup of the flour. Pulse until the nuts are finely ground, but not so long that they become pasty.
To make the cookies: In a large mixing bowl, beat together the butter and sugars until fluffy. Add the egg and extracts and beat to combine, scraping the sides and bottom of the bowl. Mix in the salt, remaining flour, and nut/flour mixture to form a stiff dough. Pat the dough into a rectangle about 1/2" thick on a piece of parchment or plastic wrap, then wrap and chill for 30 minutes.
While the dough is chilling, preheat the oven to 350°F. Line two baking sheets with parchment, or grease lightly.
To shape: Roll the dough 1/4" thick. Cut out 2" circles, and place on the prepared baking sheets. Bake for 15 to 20 minutes, until light golden brown around the edges. Remove from the oven and cool on the sheets for 5 minutes before transferring to a rack to finish cooling completely.
To assemble: When all the cookies are baked and cooled, turn half of them over and spread a generous 2 teaspoons of dulce de leche on the bottoms. Top with the remaining cookies. If desired, roll the edges of the cookies in almond flour to finish.
Store in an airtight container at room temperature for 1 week; or in the freezer for 6 weeks.
