This starts with an French Meringue to make the cookies then Italian buttercream made with maple syrup instead of sugar to fill.
Macaron shells
Preheat oven to 325 degrees.
Combine confectioner's sugar and almond meal in the bowl of a food processor and pulse several times until combined and powdery. Sift the mixture into a medium-sized bowl using a tamis or strainer. Use a flexible scraper or rubber spatula to force as much of the mixture as you can through the tamis. You should only have about 1 tsp. of the mixture left in the strainer, which can be discarded.
Make the meringue: In the bowl of a stand mixer fitted with a whisk attachment whip the egg whites with the salt on medium speed until foamy. Reduce the speed to low and stream the granulated sugar into the bowl with the mixer running. When the sugar has been added, turn the speed up to high and whisk the meringue until it holds firm, shiny peaks. Add the brown food coloring at this point, a little at a time and adjust to desired color.
With a flexible spatula, fold the almond mixture into the egg whites until completely incorporated, scraping along the sides and then up and under the middle of the bowl several times. Avoid leaving streaks of meringue or unincorporated almond mixture – this can lead to cracking in the macaron shells. When fully incorporated the mixture should have the consistency of magma – if you form a peak, it should slowly sink back into the batter. Avoid over-mixing the batter or it will become too runny and the shells will not rise properly.
Using a piping bag fitted with a small (3/8-inch, 1 cm) round tip, pipe the macarons in small circles, about 1.25-1.5 inches in diameter. Rap the baking sheets a couple of times on a flat surface, and let the piped batter sit for 15-30 minutes. When the cookies are ready there should be a small "skin" on the cookie (your fingertip should come away clean after touching one).
Bake, in a 325°F oven for 10 to 12 minutes. Use a wooden spoon to keep the oven door slightly ajar (this lets steam escape and prevents cracking), and rotate the baking sheet after 5-6 minutes for even baking.
Remove macarons from oven and transfer parchment to a cooling rack. When cool, slide a metal offset spatula or pairing knife underneath the macaron to remove from parchment.
Pair macarons of similar size, and pipe about 1/2 tsp of the filling onto one of the macarons. Sandwich macarons dust the edges of the macarons with the toasted walnuts. Refrigerate to allow flavors to blend together. Bring back to room temperature before serving.
Salted Maple Buttercream
Bring the maple syrup to a boil in a saucepan over medium-high heat (use a medium size pan as the syrup will boil). Clip a thermometer to the pan and cook until the mixture reaches 238-240°F.
While the maple syrup is cooking, whisk the egg whites and a pinch of salt on medium speed in the bowl of a stand mixer until they form medium firm peaks, about 2-3 minutes. Once the maple syrup reaches 238°F, remove from the heat and gradually and steadily pour it down the side of the bowl, resting the edge of the pan on the mixer bowl so that the syrup does not hit the whisk attachment and splatter.
Whisk the meringue until stiff peaks form and the bowl cools down, about 8 minutes. Swap out the whisk attachment for the beater. Beat on medium-high speed, adding the salt and then the butter piece by piece until incorporated and fluffy (Note: start with 1/4 tsp. sea salt to start, adding more to taste). When the beater begins making a slapping sound the buttercream is done. The buttercream can be kept covered and refrigerated for up to 2 weeks.
