Put 3/4 cup granulated sugar, 2 tbsp. fresh lemon juice, and 3 large egg whites in the bowl of a stand mixer.
Set bowl over a pan filled with 1 in. simmering water. Whisking constantly, heat mixture until egg whites are warm to the touch and sugar has dissolved, about 4 minutes.
Put bowl of warm whites on mixer and mix on medium speed until cool and fluffy, about 10 minutes. Stir in finely shredded zest of 1 lemon.
Fit a gallon-size resealable plastic bag with a 1-in. star tip, snipping hole in corner of bag for tip to fit snugly (or use a pastry bag).
Half-fill bag with meringue. Gather bag at top and gently pipe about 2 tbsp. meringue onto flat side of 1 cookie. Sandwich with another cookie. Repeat with remaining cookies. Dust cookies on both sides with powdered sugar. Makes 50.
Make-ahead tip: Chill dough (step 3) up to 1 week. Store baked cookies airtight up to 3 days. Cookies can be filled up to 5 hours ahead.