The traditional almond cookie, beloved by pastry connoisseurs the world over, is "un-refined" a little in this rustic, earthy rendition. (Shell recipe has been adapted from "Sugar Baby" (2011) by Gesine Bullock-Prado) —PieceOfLayerCake
Line two sheet pans with parchment paper, lightly spray with nonstick spray and set aside. Combine the walnuts, confectioners' sugar and meringue powder in the bowl of a food processor and blend until the mixture has completely broken down into a very fine meal, 2 - 3 minutes, stopping to scrape the work bowl a couple times with a rubber spatula. Sift the meal into a large bowl, through a fine-mesh strainer, returning any fragments that didn't pass through back to the food processor. Repeat the process until all of the ingredients are successfully sifted (a few little bits remaining in the strainer are ok). Add the salt and toss lightly to combine.
Combine ¼ c. of the water and the granulated sugar in a small saucepan and set over medium heat. Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment. When the sugar has dissolved and the mixture begins to bubble, attach a candy thermometer to the side of the pan. When the temperature reaches 230°F, begin mixing the egg whites on medium speed. As the temperature on the sugar syrup reaches 240°F, remove it from the heat. Raise the speed on the egg whites to medium-high and beat until the meringue achieves soft peaks. Lower the speed to medium-low and slowly stream in the hot sugar syrup. Attempt to pour the sugar syrup as close to the edge of the meringue as possible, without coating the side of the bowl, and without hitting the whisk as it beats. When all of your sugar syrup has been added, crank the mixer speed to high and beat until the meringue achieves stiff peaks, and the bowl is barely warm to the touch.
Add the remaining ¼ c. water to the walnut mixture and stir with a spatula until you have a uniform paste. Take ½ c. or so of the stiff meringue and vigorously stir (I use my hand) it into the walnut mixture. You're looking to lighten the stiff paste up a bit, so the rest of the meringue will be easier to incorporate. Using the rubber spatula, gently fold in the rest of the meringue, a little at a time, until you have a batter, that falls into silken ribbons on itself then gradually disappears into the mass (You may not need all of the meringue for this).
Using a pastry bag, fitted with a large round tip, pipe the macaron batter into quarter-sized rounds, spaced ½" apart, onto the prepared sheet pans. Lift the pans and give them a few, quick taps on the counter to settle the texture and dislodge any air bubbles (If you notice you're left with any peaks on the shells, you can gently press them down with a wet finger). Set the sheet pans aside to rest, at room temperature for at least 20 minutes. Preheat the oven to 275°F (If you have a convection feature on the oven, now would be a good time to use it).
Bake the shells for 12 - 15 minutes, rotating the pans once for even baking. Remove them from the oven and allow to cool completely on the pans. Unfilled shells keep well in an airtight container, for 2 weeks. Return to room temperature before filling and serving.
To make the buttercream, combine the butter and confectioners' sugar in the bowl of a stand mixer fitted with the paddle attachment and beat on medium-low speed until combined. Raise the speed to medium-high and beat until the mixture is smooth and creamy. Add the maple syrup and salt and beat on medium-high speed until the buttercream is rich and fluffy.
To fill the cooled macaron shells, pipe a dime sized dollop of buttercream onto one shell, top with another and very gently press them together. Serve immediately. Leftover filled macaron keep airtight, at room temperature, for 1 - 2 days max.