Prepare the macaron batter first. Place the almond flour, (optional all-purpose flour), and confectioner's sugar into a fine mesh sifter and sift the flour and sugar into a large bowl. Sift through twice if desired for a more smooth finish on the macarons.
Place the eggs white and granulated sugar into a mixer bowl and whisk on high speed for 3 to 4 minutes, just until stiff peaks form; don't overmix. Add the food coloring and lemon extract and mix again for about 20 seconds, just until incorporated. Transfer the egg whites into the dry ingredients. Use the 'fold and flatten' method, folding the meringue into the flour and flattening it against the mixing bowl. Fold until the batter reaches ribbon stage - it should ribbon of the spatula and settle down within 10 to 15 seconds. Watch my video to see how it's done.
Line two large baking sheets with parchment paper or a silicone mat. Transfer the macaron batter into a pastry bag tipped with a simple round tip. Pipe 1-inch dollops of the batter onto the prepared paper, spacing about 1 inch apart. After piping the entire pan, tap the baking sheet again your work surface to release any trapped air bubbles. Allow the cookies to rest at room temperature for at least an hour, for a thin shell to form on the outside.
Preheat the oven to 315F/157C and bake the rested cookies for 17 to 18 minutes. Please note - each oven bakes slightly differently and because this recipe requires precise temperature, your oven temperature and baking time might vary by a few degrees and minutes. Allow the cookies to cool on the pan.
Meanwhile, prepare the lemon buttercream: place the softened butter into a small mixing bowl and whisk on high speed for 3 to 4 minutes until the butter is light and fluffy. Add the sugar and mix again for a few minutes. Add the lemon extract, lemon zest, yellow color and citric acid last, then mix again for about a minute until the flavors are well incorporated. Transfer the buttercream into a pastry bag.
For the lemon curd, you can use a store-bought lemon curd or make a batch of my homemade lemon curd (you will have some leftover): Combine the sugar, egg and egg yolks in a small stainless steel pot and whisk vigorously for 2 to 4 minutes, until the mixture is pale yellow in color and ribbons off the whisk. Whisk in the lemon juice, then cook the mixture over medium heat, stirring constantly with a rubber spatula. Cook for 4 to 5 minutes, until the curd thickens and starts to bubble lightly. Remove from heat immediately. Add the unsalted butter and vanilla and whisk until the butter is completely melted. Allow the lemon curd to cool in the refrigerator, covered, until chilled.
To assemble the cookies, first allow them to cool completely. Pair the cooled cookies by size. Pipe a ring of the lemon buttercream on to one half, then add the lemon curd into the center. Watch my video to see how it's done. Gently sandwich the cookie halves together. Place the lemon cookies into an airtight container and into the refrigerator for at least 1 day, to allow the cookies to mature. Remove the cookies from the fridge about 30 minutes before enjoying.
These lemon macarons will also freeze well. Simply place them into a sealed container; they will keep for 6 months in the freezer.
NOTE: If you don't mind gluten in the recipe and you're really struggling to get a perfect macaron, try adding 1 tbsp of all-purpose flour to the almond flour. It helps keep the macarons full inside and helps keep their shape.