Here, a crunchy cookie shell with a chewy interior sandwiches delicate, creamy, floral frosting. If it will help, you can print out a piping template online and place it under the parchment paper to pipe consistent size circles. You can, if you like, garnish the tops of the macaroons with fresh elderflowers, which are available from marxfoods.com. The macaroons will keep, in an airtight container, for up to a few days.
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.
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.
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.
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.
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.
Gently remove cookies from parchment paper. Pipe about 1 tablespoon of frosting onto each of 24 cookies; top with remaining cookies.