1. Preheat oven to 200 degrees F. Line a baking sheet with parchment paper; set aside. Place egg whites and sugar in a double boiler or 2-quart saucepan set over a larger pot of simmering water. (The upper pan should not touch the water.) Cook, whisking constantly, until the sugar is dissolved and an instant-read thermometer registers 130 degrees F., about 6 minutes.
2. Remove pan from the heat; add vanilla and lemon if using. With an electric mixer on medium speed, beat the eggs until stiff peaks form, about 12 minutes.
3. Transfer meringue to a pastry bag fitted with a 1/4" plain round tip. Pipe a 2-inch long "I," a heart, and a "U" onto parchment paper. Repeat with remaining batter. Bake for 45 minutes or until meringues appear dry and firm when lightly touched. Cool on a baking sheet for 5 minutes. Carefully transfer cookies to a wire rack. Cool completely.