Preheat your oven to 300°F (150°C). Line two baking sheets with parchment paper or silicone baking mats.

In a bowl, sift together the almond flour and powdered sugar to create a fine mixture.

In a separate bowl, whisk the egg whites until soft peaks form. Gradually add the granulated sugar and cream of tartar, then continue whisking until stiff peaks form.

Gently fold in the sifted flour mixture and lime zest into the egg whites. If desired, add a few drops of green food coloring.

Transfer the mixture to a piping bag fitted with a round tip. Pipe small circles onto the prepared baking sheets, ensuring they are evenly spaced.

Allow the piped macarons to sit at room temperature for 30-60 minutes, until they develop a slight skin.

Bake the macarons for about 15-18 minutes, or until they look dry and have risen. Let them cool completely before removing.

In a bowl, cream the softened butter until smooth. Gradually add the powdered sugar and lime juice, mixing until creamy.

Turn half of the cooled macaron shells upside down and pipe a dollop of lime buttercream onto each one. Place the other half on top and press gently.

Allow the assembled macarons to sit for a few hours for the flavors to meld together before serving.