Preheat your oven to 300°F (150°C). Line two baking sheets with parchment paper, and if you desire, use a template to pipe uniform circles for the macarons.
In a medium bowl, sift together the almond flour and powdered sugar to ensure there are no lumps. Set aside.
In a clean, dry mixing bowl, combine egg whites and a pinch of salt. Using an electric mixer, beat the egg whites on medium speed until frothy, then gradually add granulated sugar. Increase the speed to high and whisk until soft peaks form. Add yellow food coloring if desired and whisk until stiff peaks form and the mixture shimmers.
Gently fold the sifted almond flour mixture into the whipped egg whites using a spatula. Be careful not to deflate the meringue. Fold until the mixture flows like lava and forms a figure-eight when lifted.
Transfer the macaron batter to a piping bag fitted with a round tip. Pipe small circles onto the prepared baking sheets, leaving space between each. Tap the baking sheets gently on the counter to remove air bubbles.
Let the piped macarons sit at room temperature for 30-60 minutes, or until a skin forms on the surface that doesn't stick to your fingers.
Bake the macarons in the preheated oven for 15-20 minutes, or until they have formed a “foot” and can easily be lifted off the parchment without sticking. Rotate the sheets halfway through for even baking.
Allow the macarons to cool completely on the baking sheets before removing them.
In a bowl, beat the softened butter until creamy. Gradually add in the powdered sugar, lemon juice, and lemon zest, mixing until smooth and fluffy.
Pair macaron shells of similar sizes. Pipe about a teaspoon of lemon filling onto one shell and gently sandwich it with the other. Repeat for the remaining shells.
Place the assembled macarons in the refrigerator for at least 24 hours to allow flavors to meld, then bring them to room temperature before serving.
Notes
Chill assembled macarons for at least 24 hours for best flavor.