In a medium heatproof bowl, whisk together egg yolks, granulated sugar, Key lime juice, and Key lime zest until well combined.

Place the bowl over a saucepan of simmering water (double boiler setup), ensuring the bottom of the bowl does not touch the water. Cook, whisking constantly, until the mixture thickens enough to coat the back of a spoon (about 8-10 minutes) and reaches 170°F (77°C).

Remove from heat and immediately whisk in the cold butter pieces, one at a time, until fully incorporated and smooth. Strain the curd through a fine-mesh sieve into a clean bowl to remove any zest or cooked egg bits.

Cover the surface of the curd directly with plastic wrap to prevent a skin from forming. Chill in the refrigerator for at least 2 hours, or until firm.

Measure and sift together the almond flour and 120g powdered sugar into a large bowl. Ensure there are no lumps. Set aside.

In a small saucepan, combine 120g granulated sugar and 35g water. Heat over medium heat, stirring occasionally, until the sugar dissolves. Insert a candy thermometer and continue to cook without stirring until the syrup reaches 248°F (120°C).

While the syrup cooks, begin whipping 45g aged egg whites in the bowl of a stand mixer fitted with a whisk attachment on medium speed until soft peaks form.

Once the sugar syrup reaches 248°F (120°C), immediately remove it from heat. With the mixer on medium-low speed, slowly and carefully pour the hot syrup in a thin stream down the side of the bowl into the whipping egg whites. Increase speed to high and whip until the meringue is glossy, stiff, and has cooled to lukewarm (about 8-10 minutes). Add green gel food coloring, if using, and whip for another 30 seconds until evenly distributed.

Add the remaining 45g aged egg whites to the almond flour and powdered sugar mixture. Mix with a rubber spatula until a thick paste forms (the 'tant pour tant' paste).

Add about one-third of the Italian meringue to the almond paste mixture. Gently fold until just combined. Add the remaining meringue in two additions, folding carefully until the mixture reaches the 'macaronage' stage. This means the batter flows slowly like lava from your spatula, forming a ribbon, and you can draw a figure 8 without the ribbon breaking. Be careful not to overmix.

Transfer the macaron batter to a piping bag fitted with a round piping tip. Line baking sheets with macaron silicone mats or parchment paper with drawn circles (approx. 1.5 inches).

Pipe uniform circles of batter onto the prepared mats. Once piped, firmly tap the baking sheets several times on the counter to release any air bubbles. Use a toothpick to pop any remaining visible bubbles.

Let the piped shells rest at room temperature for 30-60 minutes, or until a skin forms on the surface. When lightly touched, the batter should not stick to your finger. This is crucial for developing 'feet'.

Preheat your oven to 300°F (150°C).

Bake one sheet at a time for 12-15 minutes, rotating halfway through. Macarons are done when they have developed 'feet' and do not wiggle on their 'feet' when gently touched.

Remove from oven and let cool completely on the baking sheet before attempting to peel them off the mat. If they stick, they are not fully cooked or cooled.

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

Gradually add the sifted powdered sugar, one cup at a time, beating until fully incorporated after each addition. Scrape down the sides of the bowl as needed.

Add the Key lime juice and Key lime zest. Beat on medium-high speed for another 2-3 minutes until the buttercream is light, fluffy, and well combined. Adjust juice/zest to taste if desired.

Match macaron shells into pairs of similar size.

Transfer the Key lime buttercream to a piping bag with a round tip. Transfer the chilled Key lime curd to a separate small piping bag or use a spoon.

Pipe a ring of Key lime buttercream onto the flat side of one macaron shell, leaving a space in the center.

Fill the center space with a small dollop of the Key lime curd.

Gently place the second macaron shell on top and press lightly to sandwich the filling. Repeat with remaining shells.

Allow the macarons to 'mature' in an airtight container in the refrigerator for at least 24 hours before serving. This allows the flavors to meld and the shells to soften slightly for the best texture.