1. Combine egg whites, lemon juice, and vanilla in the bowl of an electric mixer. Whip, first on low speed, then slowly progressing to high speed, until soft peaks form, about 1 1/2 minutes.

2. Slowly sprinkle the sugar a little at a time over the egg-white mixture, and whip until glossy peaks are formed, about 3 minutes.

3. Sift the cornstarch over the meringue, and fold in. Use meringue immediately.