Line baking sheets with parchment paper or silicone macaron mats. Weigh every ingredient in grams before beginning, since small measurement differences can affect the shell texture.

Sift the almond flour and powdered sugar together, reprocessing only large pieces that will not pass through the sieve. Stir in the finely grated zest of two limes.

Whip the egg whites on medium speed until foamy, then gradually add the caster sugar. Continue whipping on medium-high until the meringue is glossy and forms stiff peaks, adding gel coloring near the end if desired.

Fold the dry ingredients into the meringue in several additions. Stop when the batter flows slowly from the spatula in a thick ribbon and blends back into itself within roughly 10–20 seconds.

Pipe rounds approximately 3–4 cm in diameter onto the prepared trays. Tap each tray firmly against the counter several times to release large air bubbles, then pop visible bubbles with a toothpick.

Leave the piped shells at room temperature for 30–60 minutes, or longer in a humid kitchen, until the surface feels dry and does not stick to a fingertip.

Bake one tray at a time at 150°C/300°F conventional heat, or about 140°C/285°F fan heat, for approximately 12–16 minutes. Cool the shells completely before removing them from the parchment or mat.

Beat the softened butter until smooth, then gradually add the sifted powdered sugar. Mix in the lime juice, lime zest, salt, and optional vanilla, beating until light and fluffy.

Pipe a ring or mound of buttercream onto half of the cooled shells. Add the remaining shells and twist gently to sandwich them without pressing straight down.

Place the filled macarons in an airtight container and refrigerate for at least 12 hours, preferably 24 hours. Bring them to room temperature for about 20–30 minutes before serving.