Line two baking sheets with parchment and fit a pastry bag with a ½-inch plain tip or coupler; set aside.
Blend almonds in a food processor until finely ground. Add 1¾ cups confectioners’ sugar and 2 tsp zest; pulse to combine.
In a separate bowl, whip egg whites and cream of tartar to soft peaks. Sprinkle in granulated sugar and whip to stiff peaks.
Fold almond mixture into egg whites in two batches. Transfer batter to pastry bag and pipe 1-inch rounds an inch apart on prepared baking sheets; let sit 1 hr at room temperature.
Heat oven to 300ºF with two racks centered. Bake, rotating position after 10 min, until cookies are golden, about 20 min. Cool completely before removing from parchment.
Make filling: Beat together butter, ½ tsp zest and juices. Add ¾ cup confectioners’ sugar and beat until fluffy, 1 min. Pipe a small amount on the bottom side of half the cookies; sandwich with the other cookies
The recipe calls for letting the cookies rest for an hour before baking. Some bakers swear by this practice for macarons, but others bake right away. What resting does is creates a skin or crust on the top of the cookie. It dries out just enough so that when baked, the air bubbles escape through the bottom of the cookie creating "feet." The air bubbles don't come out of the top of the cookie so it looks shiny and smooth.