Elderflower Macarons

Directions

1. 1
   Sift together almond meal and 1 3/4 cups of the powdered sugar until thoroughly combined and aerated, about 3 times, discarding any larger chunks. Set aside.
2. 2
   Beat egg whites with an electric mixer on medium speed until foamy, about 1 minute. Gradually add granulated sugar and cream of tartar, beating until mixture is glossy and holds soft peaks, about 4 minutes. If desired, add 1 to 2 drops of food coloring, beating just until incorporated. Gently fold almond meal mixture, in thirds, into egg white mixture. Gently fold until batter falls in a steady ribbon when spatula is lifted out of mixture.
3. 3
   Transfer batter to a piping bag fitted with a 1/4-inch round tip, and pipe 1-inch rounds on parchment paper-lined baking sheets, leaving 1 1/2 inches between each cookie. Gently tap baking sheets on the counter 4 times to release air bubbles. Set aside until tops of cookies are dry, about 30 minutes.
4. 4
   While cookies are drying, preheat oven to 350°F. Place 1 baking sheet of cookies on bottom oven rack of preheated oven, and immediately reduce temperature to 300°F. Bake cookies 16 minutes. Crack oven door open, and continue baking with oven door ajar until edges of cookies can be gently lifted from parchment, 5 to 8 minutes. Cool completely on baking sheet, about 20 minutes. Repeat process with remaining cookies, making sure to return oven temperature to 350°F before placing baking sheet in oven and reducing temperature to 300°F.
5. 5
   Beat butter on medium-high speed until creamy, about 2 minutes. Reduce speed to medium-low; gradually add remaining 2 cups powdered sugar, and beat until completely incorporated. Stir in liqueur and salt. Increase speed to medium-high, and beat until mixture is light and fluffy, about 5 minutes. Transfer to a piping bag fitted with a round tip.
6. 6
   Gently remove cookies from parchment paper. Pipe about 1 tablespoon of frosting onto each of 24 cookies; top with remaining cookies.