Preheat oven to 300 degrees
Beat egg whites, cream of tartar and sugar for 8-10 mins.
Whip until they form a peak that stands upright.
Then add the food coloring. Color does fade as it cooks, so do a shade or two darker than you want them to be.
Sift almond flour, and powdered sugar. What remains will be the larger lumps of almond pieces. Just discard those, or use them to snack on You want a really fine powder mixture to create a smooth and pretty on top to your cookie.
Fold flour/sugar mixture into the egg white mixture.
This is where all your hard work can really go wrong. Approximately 65-75 turns of your spatula when folding is about the right amount of time. But again, it can be tricky,
Transfer batter to a pastry bag.
Pipe out 1 inch rounds on a baking sheet lined with parchment paper.
Tap the pan hard at least 2-3 times to release the air bubbles. This will prevent the tops of your macaroons from cracking.
Let them sit out for 30-40 minutes, or up to an hour if you want. This will allow them time to dry out a bit before hitting the hot oven. They should be "tacky" to the touch, but not stick to your fingertips. This is another important step to assuring your macaroons develop feet! When they dry out they can't spread out in the oven, and are forced to rise up. That's what creates the feet!
Bake for 20 mins. DO NOT UNDER BAKE, even if they look done! Otherwise they will stick to your tray.
Meanwhile mix the buttercream.
Whip butter with an electric mixer until pale and fluffy. Slowly add sugar.
Add juice to buttercream, and whip until combined. Transfer to a pastry bag, fitted with a small tip (about ¼ " in diameter)
Reverse cookie shells on their backs, and pipe a small mound of filling on one of them. Top with the other shell et Voila!