Make dough: Beat butter and brown sugar together in a bowl with a mixer on medium speed. Mix in molasses, then egg, until blended, scraping bowl as needed.
In a small bowl, combine flour, baking soda, salt, and spices; add to butter mixture on low speed, mixing until combined.
Divide dough in half, shape each into a disk, wrap in plastic wrap, and chill until firm, about 3 hours.
Preheat oven to 350°. Unwrap dough. On a generously floured surface, roll out each disk until 1/8 in. thick. Using snowflake cutters to cut out dough in various sizes, some with the centers cut out using a smaller size cutter to make a window, and arrange snowflakes about 1 in. apart on parchment-lined baking sheets (use a small spatula to transfer). Reroll scraps as needed.
Bake cookies until dry-looking and just starting to brown on edges, about 8 minutes. Set pans on racks and let cookies cool on pans.
Make meringue: Put sugar, lemon juice, and egg whites in the bowl of a stand mixer. Set bowl over a pan filled with just enough simmering water so that bowl isn't touching 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 whisk on medium speed until cool and fluffy, about 10 minutes. Stir in zest.
Fit a gallon-size resealable plastic bag with a 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.
Make ahead: Chill dough (step 3) up to 1 week. Store baked cookies airtight up to 3 days.
Note: Nutritional analysis is per cookie.