Place cranberries, zest (if using) and water in a small saucepan. Simmer over medium heat until cranberries split and soften.
Press cranberries through a wire sieve. Discard skins left behind, let pulp cool to room temperature.
Combine cranberry pulp, sugar, vanilla, and salt together in a large bowl, set aside.
In a small bowl, sprinkle gelatin over water and allow to soak for 5 minutes.
Transfer gelatin bowl to microwave, heat in 10-second increments until gelatin melts.
Pour melted gelatin into cranberry pulp mixture, stir until well incorporated, then set aside while preparing the rest of the ingredients.
In a separate bowl (stand mixer, ideally!), whip egg whites until stiff peaks form. Carefully fold into cranberry mixture, stirring until combined.
Whip cream until stiff peaks form, then carefully fold into the cranberry mixture, stirring until combined.
Pour into 4-6 serving glasses, chill until set, about 2 hours.
Serve with a dollop of whipped cream, if desired