Add the almond flour, powdered sugar, cinnamon, nutmeg, and salt into a food processor and pulse it until it is ultra fine.
Then, add the egg whites into the bowl of a stand mixer fitted with the whisk attachment. Avoid getting any yolk or water into the bowl. Begin beating the egg whites on medium-high speed. Once they start to froth, add the sugar, one tablespoon at a time and continue beating the egg whites until you get stiff peaks.
Turn off and remove your bowl from the stand mixer. Add ⅓ of the almond flour mixture and use a flexible rubber spatula to gently fold it into the egg whites. Then, add another ⅓ of the almond flour, followed by the remaining third, folding until fully combined each time.
Transfer the batter into a piping bag fitted with a round piping tip. No piping bag? Use a zip-loc with a hole cut in the corner. Pipe 1½" circles onto the baking sheets (or follow a macaron baking mat), spacing them one inch apart. Tap the baking sheet on a flat surface a few times to release any air bubbles. Sprinkle the top with some ground cinnamon if desired.
Let the macarons sit at room temperature for 45 minutes-1 hour, or until the tops are dry. This will help form the "feet". While they rest, preheat the oven to 325°F. Bake the macarons for 14-15 minutes, rotating the baking sheets halfway through. Let the macarons cool completely before frosting
To make the buttercream filling, start by beating the butter and salt together on medium speed until fluffy. Then, turn off the mixer and add the remaining ingredients. Start mixing on low speed and gradually increase to medium-high speed and beat until light and fluffy.
Transfer the buttercream to a piping bag fitted with the tip of your choosing. Assemble the macarons by piping a circle of buttercream onto the bottom of one shell, and then sandwiching with another.