Add the egg whites to a mixing bowl and beat until they begin to foam. Add the cream of tartar and sugar and continue to beat until stiff peaks form, about 4-5 minutes.
Add the food coloring and mix lightly until they color is evenly incorporated.
Macaronage
Sift the almond flour and powdered sugar into a medium bowl, discarding any pieces that do not sift.
Pour the flour and powdered sugar mixture into the egg white mixture. Use a spatula to fold the dry ingredients into the meringue. After about 10-15 folds, begin lightly pressing downward on the batter to remove any excess air. Stop folding when the batter falls from the spatula in a thick ribbon with the consistency of lava. It should be glossy with no visible lumps.
Piping
Add the batter to a piping bag and pipe 1 ½-inch rounds on a baking sheet.
Let the rounds sit for 45-60 minutes. They should appear slightly dry on the outside. Preheat the oven to 300F/150C about 10-15 minutes before the end of the resting period.
Baking
Bake the rounds at 300F/150C for 15-16 minutes. Allow the baked rounds to cool for at least 5-10 minutes before attempting to remove them from the baking sheet.
Lavender Buttercream
Heat the heavy cream until lightly steaming. Add the lavender, and allow it to steep for at least 10 minutes. Allow the mixture to cool completely before using.
In a separate bowl, beat the butter until light and fluffy (about 3 minutes). Add the powdered sugar and salt and mix until smooth.
Pour the infused lavender cream through a mesh sieve into the butter mixture, ensuring that no lavender flowers end up in the final mixture. Mix until combined.
Final Assembly
Transfer the buttercream to a piping bag and pipe about 2 tsp between each pair of macaron shells.
Chill the macarons for two hours or more to allow the buttercream to solidify before serving.