The day before you make the macarons, separate the egg whites and place them in an airtight container in the fridge.
On baking day, bring the egg whites to room temperature (by leaving them on the counter for 30 minutes) and soften the butter.
Weigh out and sift your almond flour and powdered sugar. (Do not skip this step!) Whisk them together in a large bowl. Set aside.
Using a hand mixer, whip the egg whites until frothy. Add a pinch of cream of tartar and continue to whisk. When the egg whites become white and smooth, add the cane sugar one tbsp at a time to make a meringue. Add in the food coloring with a toothpick when you’ve achieved soft peaks and continue to whisk until stiff peaks form.
Using a rubber spatula, fold the almond flour mixture ⅓ at a time into the meringue. Fold gently but make sure the flour is well incorporated into the meringue. When ready, the batter should flow continuously off your spatula long enough for you to draw a figure 8 with it in the bowl.
Fill the batter into a piping bag with a round tip and pipe the shells onto a tray lined with parchment paper. When finished, tap the tray into the counter a few times to get rid of air bubbles. Set the tray aside for 45 minutes to an hour for the macarons to “dry.” They should lose their shine and feel dry to the touch when ready.
Preheat the oven to 300F and bake for 12-15 minutes. At the 7 minute mark, rotate the tray to ensure an even bake and cover the macarons with foil if you notice slight browning.
When they’re done baking, let the shells cool in the oven with the door propped open with a wooden spoon.
In the meantime, make the filling. Cream the butter and powdered sugar together. Add in the lavender simple syrup and crushed lavender leaves. Start with ½ tsp and taste. Add another ½ tsp if you prefer a stronger lavender taste.
When the shells have cooled, pipe the filling onto one shell and sandwich with another. Place the assembled macarons in the fridge and try to resist the urge to eat them right away. They will taste their best in 2-3 days, when the filling has had time to soften the shells.