Ingredients
1 tablespoon root beer concentrate (or flavoring, plus 1 Tbsp. if needed to reach desired flavor)
2 cups confectioners' sugar (for filling)
1 cup butter (softened)
1 tablespoon superfine sugar
1/8 teaspoon cream of tartar
2 large egg whites (room temperature)
1 1/4 cups confectioners' sugar (divided, for macarons)
2/3 cup almond flour
Step by step
Place almond flour and 1 cup confectioners’ sugar in a KitchenAid Food Processor; pulse until thoroughly mixed to ensure almond flour is very fine. Pass almond flour mixture through a fine-mesh sieve; discard any large pieces that remain.
Place egg whites and cream of tartar in a very clean large bowl of a KitchenAid Stand Mixer. Using the Wire Whip Accessory, whisk on medium-low speed until frothy. Slowly add superfine sugar; whisk until dissolved, 1-2 minutes. Slowly add remaining 1/4 cup confectioners’ sugar; increase speed to high and whip until meringue is glossy and stiff peaks form, 1-2 minutes.
Gently fold almond flour mixture into meringue, a third at a time. Using the side of a spatula, smooth batter up side of bowl several times to remove air bubbles and ensure there are no lumps; do not overmix. Run spatula down the center of the bowl; the line in the batter should remain visible for a moment before mixture runs back into itself.
Position rack in upper third of oven; preheat oven to 300°F. Transfer batter into a pastry bag fitted with a #7 or #10 round tip. Pipe 1 3/8-inch rounds onto a parchment-lined tray about 1 inch apart. Tap tray against counter 2-3 times to remove excess air bubbles. Let macarons rest until no longer wet or sticky to the touch, 30-60 minutes.
Bake, one tray at a time, until cookies rise about 1/8 inch to form "feet," 14-16 minutes, rotating tray halfway through baking. Remove tray and let macarons cool completely. Repeat with remaining trays. Once macarons have cooled completely, remove from parchment.
To make filling, in a KitchenAid Stand Mixer using the Wire Whip Accessory, beat butter until light and fluffy, 3-5 minutes; slowly add confectioners' sugar until incorporated. Beat in concentrate.
Spoon frosting into a pastry bag fitted with a small round tip; pipe buttercream onto the bottoms of half the macarons. Top with remaining macaron shells. Decorate as desired. Cover and refrigerate overnight. Bring to room temperature before serving. Store in the refrigerator
Tip: Eggs separate more easily when cold, so do that when you take them out of the fridge, then let them come to room temperature.