Chilled Key Lime Mousse

Pour water into a small heatproof glass bowl and sprinkle with gelatin. Let stand until softened, about 3 minutes. Heat about 1 inch of water in a small skillet over low heat. Place the bowl in the pan and stir until the gelatin has dissolved and the liquid is clear. (Alternatively, microwave the gelatin mixture, uncovered, on High until the gelatin has completely dissolved but the liquid is not boiling, 20 to 40 seconds.) Set aside.

Combine 1 cup sugar and lime zest in a food processor and process until the zest is finely ground. Transfer to a large saucepan and add lime juice and salt. Heat over medium heat until very hot, whisking until the sugar is dissolved. Remove from the heat. Whisk egg yolks in a medium heatproof bowl. Whisking constantly, very slowly add the lime juice mixture, beginning with just a few drops and gradually increasing the amount.

Return the mixture to the saucepan. Cook over medium heat, whisking constantly, until the custard is thick enough to coat the back of a wooden spoon, 2 to 4 minutes. Immediately transfer to the bowl of a stand mixer (or a large deep bowl if using a hand mixer). Beat starting on low speed and increasing to medium-high until very thick and pale, 3 to 5 minutes. With the mixer running, slowly dribble in the gelatin mixture and continue to beat on medium-high speed until the mixture cools to room temperature, about 5 minutes. Clean the beaters.

Beat egg whites in another mixing bowl on medium speed just until foamy. Increase speed to medium-high and sprinkle in the remaining 1/2 cup sugar, about 1 tablespoon at a time, beating well after each addition. Beat just until the egg whites hold stiff peaks when the beaters are lifted.

With a whisk, fold yogurt into the lime custard. Then fold in the egg whites in 3 batches. Evenly divide the mixture among ten 8-ounce ramekins (or transfer to a 2 1/2-quart round dish, such as a souffle dish).

Refrigerate until thoroughly chilled and set, at least 3 hours and up to 24 hours. Garnish with more lime zest before serving, if desired.