Place egg whites in a small bowl (make sure there is no trace of egg yolk). Cover with plastic wrap and poke a few large holes in it. Leave out at room temperature for 8 hours or in the refrigerator overnight. Bring to room temperature before using.
Place confectioners' sugar and fine almond flour into a fine mesh sieve over a bowl. Sift and let the bowl catch the sifted ingredients. Discard any large clumps in the sieve. Repeat twice and set aside dry ingredients.
Put egg whites in a very clean bowl of a stand mixer fitted with a whisk attachment. Whisk eggs on medium-low speed for a minute or two until frothy then add the cream of tartar and salt. With the mixer running, very slowly add the bakers sugar (either 2 teaspoons at a time or in a very slowly sprinkle into the bowl). Increase speed to medium-high and beat until stiff peaks form, about 5 minutes on KitchenAid Stand Mixer speed 6. When the mixture is almost done, mix in the lemon extract. Pro tip: The mixture will ball up in the whisk and the mixture will stay in the bowl if turned upside down.
Add half of the dry ingredients to the mixing bowl. Use a silicone spatula to gently fold the dry ingredients into the wet ingredients. Once mostly incorporated, add the remaining dry ingredients. Continue folding by going around the sides of the bowl with the spatula, then cutting through the center. The batter is done when it has a lava-like consistency. Do not over-mix. Pro tip: Try lifting the spatula out of the bowl and forming figure 8 with the batter without it breaking.
Transfer batter to a piping bag fitted with a round tip (about 1/2-inch in diameter). Pipe batter onto a silpat mat with macaron outlines or onto parchment paper. Pipe circles 1-inch wide and spaced 2 inches apart. Lift the baking sheet about 6 inches off the counter and drop it down. Repeat twice more, then rotate the pan 180° and bang the pans three more times (this releases air bubbles from the macarons).
Let the macarons sit at room temperature for 40 to 45 minutes and up to an hour. The macarons are ready to bake when dry to the touch (you should be able to "pet" them without leaving a mark).
Ten minutes before macarons are done drying, preheat the oven to 300° F. Bake macarons for 12 to 14 minutes, rotating the pan halfway through. Cool completely.
Filling
In the bowl of a stand mixer, beat the butter using the paddle attachment. Beat until creamy, about 2 minutes. With the mixer on low speed, gradually add in the confectioners' sugar until completely incorporated.
With the mixer on medium speed, add the lemon zest, lemon juice and a pinch of salt. Mix until combined. If necessary, add more lemon juice until desired consistency is reached.
Assembly
Match up the cooled macarons according to size.
Transfer frosting to a bag with the desired tip.Pipe a border of buttercream on the bottom of one macaron shell. Place ½ teaspoon lemon curd in the center and top with the matching shell. Repeat with remaining cookies.
Notes
Baker's sugar: To make it at home, put granulated sugar in a food processor. Grind for about 10 seconds or until fine.Lemon extract is preferred for the macaron shells. It's more concentrated and less acidic so it won't affect the texture of the batter.Weighing ingredients: Weigh the egg whites before they are dehydrated. Weigh the confectioners' sugar and almond flour after sifting.Store in an airtight container in the refrigerator for up to 5 days. Macarons are best when eaten the day they are assembled.Freeze for up to 3 months. Carefully wrap in plastic wrap and place in an airtight container. Store in the freezer and defrost in the refrigerator before serving.