Pulse the almond flour and powdered sugar together in a food processor for about 30 seconds to ensure a fine powder. Sift the mixture into a bowl and discard any large pieces. Set aside.

In the clean, dry bowl of a stand mixer, whisk the egg whites on medium speed until frothy. Add the cream of tartar. Gradually add the granulated sugar, a tablespoon at a time, while continuing to whisk on medium-high speed. Increase speed to high and beat until stiff, glossy peaks form (French meringue). Add 1-2 drops of green gel food coloring, if desired, and mix briefly until just combined.

Add half of the sifted dry ingredients to the meringue. Fold gently with a rubber spatula using a circular motion, scraping the sides of the bowl, until just combined. Add the remaining dry ingredients and continue folding until you reach the 'macaronage' stage. The batter should flow like lava off your spatula and form a 'figure 8' without breaking. Be careful not to overmix.

Transfer the macaron batter to a piping bag fitted with a round tip (like Wilton 1A). Pipe 1.5-inch circles onto silicone baking mats or parchment-lined baking sheets. Leave about 1 inch between each macaron.

Firmly tap the baking sheets on the counter 2-3 times to release any air bubbles. Use a toothpick to pop any remaining visible bubbles. Let the macarons rest at room temperature for 30-60 minutes, or until a skin forms on top and they are no longer sticky to the touch.

Preheat your oven to 300°F (150°C) during the last 15 minutes of resting time. Bake one tray at a time for 12-14 minutes, or until the macarons have developed 'feet' and do not wiggle when gently touched. Let cool completely on the baking sheet before removing.

In a small saucepan, whisk together the Key lime juice, granulated sugar, egg yolk, Key lime zest, and a pinch of salt. Cook over medium-low heat, whisking constantly, until the mixture thickens to coat the back of a spoon (approximately 5-7 minutes). Do not boil.

Remove from heat and strain the curd through a fine-mesh sieve into a small bowl to remove zest. Cover the surface directly with plastic wrap to prevent a skin from forming and refrigerate until completely chilled (at least 1 hour).

In the bowl of a stand mixer fitted with the paddle attachment, beat the room temperature unsalted butter on medium speed until light and fluffy (about 3-4 minutes).

Gradually add the sifted powdered sugar, beating on low speed until combined, then increase to medium-high and beat until light and fluffy (another 3-5 minutes).

Slowly add the chilled Key lime curd to the buttercream, beating on medium speed until fully incorporated and smooth. Scrape down the sides of the bowl as needed.

Once the macaron shells are completely cool, match them up by size. Transfer the Key lime buttercream to a piping bag fitted with a round tip.

Pipe a dollop of buttercream onto the flat side of one macaron shell, then gently top with a matching shell, twisting slightly to secure. Repeat with the remaining shells and filling.

For best flavor and texture, refrigerate the assembled macarons in an airtight container for at least 24 hours to allow them to 'mature'.