Vanilla Bean Macarons with Salted Maple-Pecan Buttercream Filling
For the Vanilla Bean Macarons:
Preheat oven to 350 degrees F. Place almonds in a food processor; process until as fine as possible. Add powdered sugar and process until well-combined and very fine.
In the bowl of an electric stand mixer whisk egg whites and granulated sugar together for 2 minutes on medium speed. Increase speed to medium-high and whisk for 2 additional minutes. Then whisk on high for 2 minutes more. The egg whites should hold stiff, glossy peaks when you lift the whisk out of the bowl.
Add the vanilla bean seeds and beat on high speed for 30 seconds. Add the almond mixture and sea salt all at once. Fold by hand with a spatula from bottom of bowl upward, then press flat side of spatula firmly through middle of mixture. Repeat just until batter flows like molten lava, about 35 full strokes.
Line two heavy baking sheets with parchment paper. Fill a pastry bag with 3 the batter. Using a 3/8-inch tip, pipe out 3/4-inch rounds of batter, spaced 1 inch apart, swirling tip off to one side so the batter does not peak. After filling baking sheets with piped rounds, tap the sheets firmly against counter two or three times to release air bubbles.
Position an oven rack in the lower third of the oven. Bake macarons, one sheet at a time, rotating halfway through, until they are risen and just set, about 13 minutes. Let cool.
For the Salted Maple-Pecan Buttercream Filling:
In a medium-size saucepan bring the maple syrup to a boil over medium-high heat (use a medium-size pan as the syrup will rise). Cook until the mixture reaches the soft-ball stage (238/240 degrees F on a candy thermometer).
While the maple syrup is cooking, in the bowl of a stand mixer whisk the egg whites and a pinch of salt on medium speed until the egg whites form medium-firm peaks, about 3 minutes.
When the maple syrup reaches 238 degrees F, remove from heat and gradually and steadily pour it down the side of the mixer bowl, resting the edge of the pan on the bowl so the syrup does not hit the whisk attachment and splatter. Whisk the meringue until stiff peaks form and the bowl feels just warm (not hot) to the touch, about 8 minutes.
Swap out the whisk attachment for the beater. Beat on medium-high speed, adding the salt and the butter, piece by piece, until incorporated and fluffy. (Note: Start with 1/4 teaspoon salt, adding more to taste.) When the beater begins to make a slapping sound, the buttercream is done.
Gently fold in about half of the finely chopped pecans (save the rest for assembling the cookies) until thoroughly mixed but not deflated. The buttercream can be covered and refrigerated for up to 2 weeks.
For the Finished Macarons:
Pipe or spread Salted Maple-Pecan Buttercream Filling onto flat sides of half the macarons; top with the remaining half.
Roll the edges of each macaron in the remaining finely chopped pecans so they adhere to the exposed filling as a pretty and tasty garnish. Wrap macarons in plastic wrap and refrigerate.