Step 1If using fresh passion fruit, cut the fruit in half and strain the pulp through a fine sieve, rubbing to remove the pulp from seeds. Wash the seeds in water, then dry them on a paper towel. They will be used to garnish the mousse.
Step 2Place 2 tablespoons puree or juice concentrate in a small, non-reactive saucepan and sprinkle the gelatin over. Stir until blended and smooth. Heat the mixture over low heat, stirring, just until the gelatin is dissolved.
Step 3Remove the gelatin mixture from the heat. Add the remaining juice and stir to combine. Add the evaporated milk and one-fourth cup sugar; stir until dissolved. Chill until slightly thickened, stirring occasionally.
Step 4Beat the egg whites and the salt in a mixing bowl to soft peaks. Gradually add the remaining one-fourth cup sugar and beat until stiff and glossy.
Step 5Add a large spoonful of the egg white mixture to the chilled passion fruit mixture and combine thoroughly. Fold in the remaining meringue with a spatula, making sure not to over-mix.
Step 6Spoon the mousse into individual margarita or stemmed glasses. Chill. Sprinkle with the reserved passion fruit seeds and serve.