I highly recommend using a kitchen scale for this recipe as the ingredients need to be precisely measured for best results. First, place the egg whites and granulated sugar into a mixer bowl and whisk on high speed for 3 to 4 minutes, just until stiff peaks form. Add in the coconut extract and whisk for another 15 seconds, just until the flavor is incorporated.
Next, prepare the dry ingredients. Sift the almond flour and confectioner's sugar into a large mixing bowl using a fine mesh sifter. Add the whisked egg whites to the bowl and using a spatula, begin coming the dry ingredients and egg whites. I use the flatten and fold method; check out my video recipe to see how it's done. Mix from the bottom of the mixing bowl, bringing up any dry ingredients, and flatten the egg whites against the sides of the mixing bowl. Keep mixing for a few minutes, until the batter reaches 'lava stage' - it will ribbon off the spatula and set down on itself in 10 seconds.
Transfer the macaron batter into a pastry bag tipped with simple, round tip; I used Ateco tip #806. Prepare two large baking sheets by lining them with parchment paper or a silicone mat. Pipe 1-inch (2.5-cm) macarons onto the prepared baking sheets, spacing them about an inch apart. Once the baking sheet is full, tap the pan hard a few times against your work surface to release any trapped air bubbles. Pop any bubbles that rise to the surface with a toothpick.
Allow the macarons to rest at room temperature for 1 hour, or until a thin film forms on the top. This process can take longer in high humidity climates. Preheat the oven to 315F/157C. Bake the rested cookies for 17 to 19 minutes, just until the edges start turning a golden color and the cookies come off the paper easily. Remove the cookies from the oven and let them cool completely.
For the Lime Buttercream: place the softened butter into a medium sized mixing bowl and beat on high speed for about 4 minutes, until the butter is fluffy and white in color. Add in the confectioner's sugar and mix first on low speed until the sugar is absorbed, then on high speed for 2 to 3 minutes, until a fluffy buttercream forms. Add in the lime extract, citric acid, lime zest and a touch of green food coloring, if desired. Mix the buttercream again for 1 minute, until everything is well combined. Transfer the buttercream into a pastry bag tipped with a small, star tip (I used Ateco tip #823).
For the Lime Curd: in a medium-sized stainless steel pot, combine the egg, egg yolk and sugar. Whisk vigorously for 2 to 3 minutes, until the eggs become thick, pale and ribbon off the whisk. Whisk in the lime juice, then take the mixture to the stove top. Cook over medium heat, stirring constantly with a rubber spatula, until the curd thickens; about 4 minutes. Remove the curd from the heat and add in the butter; whisk until the butter is completely melted. Add in the lime extract, lime zest and a bit of green food coloring. Cover the curd with plastic wrap so it touches the surface and cool the lime curd completely.
To assemble the cookies: once the cookies are cooled, pair them up by size and turn one over for the fillings. Pipe a ring of lime buttercream onto one half and fill the center of each cookie with a bit of lime curd. Gently sandwich the cookies together.
For best results: place the cookies into an airtight container and into the refrigerator. I recommend keeping these macarons refrigerated for 1 to 3 days to allow the macarons to ‘mature’. When they ‘mature’, the shells become softer and even more delicious! Take them out at least 30 minutes to an hour before enjoying to allow the cookies to thaw and come up to room temperature.