1. Line two baking sheets with parchment and fit a pastry bag with a 1/2-inch plain tip; set aside.
2. Blend almonds in a food processor until finely ground. Add 1-3/4 cups confectioners' sugar and 2 teaspoons zest; pulse to combine.
3. In a separate bowl, whip egg whites and cream of tartar to soft peaks. Sprinkle in granulated sugar and whip to stiff peaks.
4. 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 hour at room temperature.
5. Heat oven to 300 degrees F with two racks centered. Bake, rotating position after 10 minutes, until cookies are golden, about 20 minutes. Cool completely before removing from parchment.Make filling::
6. Beat together butter, 1/2 teaspoon zest and juices. Add 3/4 cup confectioners' sugar and beat until fluffy, 1 minute. Pipe a small amount on the bottom side of half the cookies; sandwich with the other cookies. Makes 32 cookies.