Espresso Macarons with Fleur de Sel Caramel Filling

Salty caramel has become ubiquitous in pastry shops and restaurants around the country. It makes a great filling for Espresso or Vanilla macaron shells, and, in a more special combination, Pink Peppercorn (find all these recipes in the cookbook). For a more pronounced fleur de sel effect, sprinkle the salt over the filling before topping it with the second shell.

1-1/4 packed cups (165 grams) almond flour
3/4 packed cup (165 grams) confectioners' sugar
Pinch fine sea salt
1 tablespoon (5 grams) instant espresso powder
1 tablespoon (5 grams) powdered egg white
3/4 cup (150 grams) granulated sugar
1/2 cup (115 grams) aged egg whites, from 4 eggs (Note: Aged egg whites are those which have been separated, lightly whisked, covered with plastic wrap into which you have poked a few holes, and refrigerated for two to five days, in order to concentrate their protein structure, for a firmer meringue.)
1/2 teaspoon (3 grams) cream of tartar
2 drops liquid yellow food coloring
Fleur de Sel Caramel Filling (recipe below)

1. Preheat the oven to 200° F. Stack two 18x13-inch baking sheets on top of one another. Line the top baking sheet with a silicone mat or parchment paper. Cut additional sheets of parchment paper, if using, at the dimensions of the baking sheet to pipe additional batches.

2. Place the almond flour, confectioners' sugar, and salt in the bowl of a food processor and pulse 4 times for 3 seconds each to combine them. Scrape the sides of the bowl in between pulses with a spatula. Sift with a fine-mesh strainer onto a sheet of waxed paper. (If you simply sift the flour and sugar together without processing them first, the macaron will not be as smooth-skinned.) Stir the espresso powder into this mixture and set aside.

3. With a hand whisk, whisk together the powdered egg whites and granulated sugar in the bowl of an electric mixer. Whisk in the egg whites and cream of tartar until the mixture is homogenous. Set the bowl and whisk attachment on the mixer and whisk on medium speed until the meringue is glossy and forms stiff peaks, about 11 minutes. Once the meringue reaches stiff peaks (the whisk will leave marks in the meringue as it goes around in the bowl) and resembles marshmallow fluff, stop the mixer. Turn the bowl upside down to check that you have reached the right stage: the meringue should not slip in the bowl.

4. With a spatula, quickly fold the sifted dry ingredients into the meringue. Stop folding when the batter appears to be 90 percent incorporated, scrape the sides of the bowl, and fold in the yellow food coloring with 1 to 2 more strokes.

5. Spoon the batter in a pastry bag fitted with a 1/2-inch round tip (alternatively, cut a 1/2-inch opening in the bag). Fill the bag halfway, leaving the rest of the meringue in the bowl while piping; cover it with plastic wrap while a batch is in the oven. If you overfill the bag, you will not be able to squeeze it hard enough to pipe even, tail-less shells. Twist the top of the bag to close. Pipe the meringue on the silicone mat into quarter-sized mounds, 1-1/2 inches apart from one another.

6. Bake the shells at 200° F for 15 minutes to dry out the shells. Increase the oven temperature to 350° F and bake for an additional 9 minutes, until the foot and edge of the shells feel firm and they just come off the silicone if you lift them. Remove the shells from the oven. Slide the silicone mat or parchment paper onto a cooling rack and let the shells cool completely, about 1 hour.

7. To fill the shells, spoon the caramel filling into a pastry bag and cut a 1/4-inch opening at the tip. Pipe a small amount of caramel, about 1/4-inch thick, on the flat sides of the shells. Don't let the caramel go all the way to the edges of the macaron. Top with another shell, twist it slightly to secure the filling, and let set in the refrigerator, about 10 minutes.

Reprinted with permission from "Les Petits Macarons" by Kathryn Gordon and Annie E. McBride, published by Running Press.