Add the egg whites to the stand mixer fitted with the whisk attachment. Blend on speed 8 for 1 minute, or until egg whites are frothy. Turn the speed to 2 and slowly pour in the white sugar. Bring the speed back up to level 8 and whisk until stiff peaks form, and the meringue is glossy and shiny.
Add 2-3 food coloring drops at a time until the desired color has been reached.
In a food processor, pulse together the almond flour and powdered sugar until smooth, ensuring no clumps.
Add half of the almond flour mixture and gently mix into the meringue until incorporated. Fold the remaining almond flour mixture into the batter, careful not to deflate the meringue. The batter should be runny enough to pipe, but not too loose it loses its shape. To test, scoop some batter onto your spatula and make a figure 8. If the batter falls in a consistent drip to create the 8 but does not lose shape, then it’s the perfect consistency.
Scoop all the batter into a piping bag fitted with a 1-inch circle tip. Line your baking sheet with a silicone baking mat, or parchment paper. Pipe equal sized circles onto the prepared baking sheets, about 1 ½ inches away from each other.
Tap the baking sheet on the counter several times to eliminate any remaining air bubbles.
Bake in the prepared oven for 18 – 20 Minutes, or until the tops of the shells are hardened. Allow the macarons to cool on the silicone baking mat before removing.
While the macarons are baking, prepare the buttercream. Add the butter to the stand mixer fitted with the paddle attachment. Whip the butter until smooth and creamy on level 8 for 3 minutes. Turn the speed to level 2 and slowly incorporate the powdered sugar, lemon juice, lemon zest, and salt. Bring the speed back up to level 8 and whip for an additional 2 minutes.
Using a small spatula or spoon, spread a teaspoon of buttercream on the bottom of a macaron, and fit together with another to make a sandwich. Continue until all shells are filled. Store in airtight containers.