First, read Sarah’s tips to know what the consistency of the macaron batter should be.
Preheat oven to 300° Fahrenheit.
Sift together almond flour and powdered sugar with a fine mesh strainer. Sift a second time and set aside.
Whisk the egg whites and granulated sugar in the bowl of a stand mixture fitted with the whisk attachment. Increase speed to medium (4 on a KitchenAid) and beat for 2 minutes. Increase speed again to medium-high (6 on a KitchenAid) and beat for an additional 2 minutes. Finally, increase the speed to high (8 on a KitchenAid) and beat for another 2 minutes. The egg whites should be very stiff and glossy, and clump together in the bowl. Beat in the gel food coloring and lavender essential oil.
Remove bowl from mixer stand. Add the sifted dry ingredients to the bowl in a single addition. Use a spatula to fold the dry ingredients into the meringue using a figure-eight motion, cutting the flat side of the spatula through the center of the batter and pressing it against the side of the bowl. Repeat until the batter reaches the consistency of molten lava, about 40-43 strokes.
Pipe the French Macarons:
Fill an extra-large pastry bag fitted with a 3/8-inch round piping tip with macaron batter. If you notice some unmixed dry ingredients in your bowl do not add them to the piping bag.
Pipe the macaron batter on to baking sheets lined with parchment paper. If the parchment paper corners curl up, use a dab of macaron batter to ‘glue’ them down. To get consistent sizes, you can use a printable macaron guide under the parchment paper. Just be sure to remove the template before baking.
Pipe the batter into a 1 1/2-inch round, swirling the tip and lifting straight up at the end of each shell, spacing the macarons at least 1 1/2 inches apart. Bang the baking sheets firmly on the counter 2-3 times to release any air bubbles. If there is a peak on the shell you can use a toothpick to smooth it. Let rest for 20-30 minutes, until the macarons are dry to the touch and the batter doesn’t stick to your finger.
Bake one sheet at a time until they have risen. Check at 15 minutes and then every 2 minutes until the tops no longer jiggle from the feet, and they lift from the parchment paper with a thin spatula or your fingernail. Let cool completely before filling with lemon buttercream.
Make the Lemon Buttercream Filling:
Bring a medium saucepan filled with one inch of simmering water to a boil. Place the egg whites and sugar in a stainless-steel bowl (like a stand mixer bowl) and place on the saucepan. Use a whisk to beat the mixture until very hot (about 160° F).
Remove from heat and mix with the whisk attachment on high until it is cool, thick, and glossy and has tripled in volume, about 5 minutes.
Reduce speed to medium and add the butter a few pieces at a time. Add the lemon zest, lemon juice, lemon essential oil (if using) and continue to beat until smooth and fluffy.*
Assemble the macarons:
Pair the macaron shells by similar size and shape. Put buttercream frosting in a piping bag fitted with a 3/8-inch round piping tip. Pipe a dollop of frosting onto half the macaron shells and gently make a sandwich with the second macaron shell.
If not eating immediately, store the macarons in an airtight container in the fridge. Flavor will improve with 2-3 days of ‘ripening’ in the fridge. If not using in that time frame, store unfrosted shells in an airtight container in the freezer for up to 3 months.
*Notes on Making & Troubleshooting Buttercream Frosting: If the frosting is too soft and doesn’t seem to be coming together, it’s possible that it’s too warm. You could try popping it into the fridge for 10-15 minutes and then re-mixing with the paddle attachment in your electric mixer until it comes together and is smooth and satiny. In general it’s hard to over-whip buttercream, so I would err on the side of mixing it longer to see if you can salvage it, even as long as 15 minutes!