Step 1If using fresh fruit, halve the fruit and strain the pulp through a fine sieve, rubbing to remove the pulp from the seeds. Wash the seeds in water, then dry on a paper towel. They will be used to garnish the mousse.
Step 2Place two tablespoons puree or juice concentrate in a small, nonreactive heavy-bottom saucepan and sprinkle over the gelatin to moisten. Heat the mixture over medium-low heat, stirring, just until the gelatin is dissolved and no lumps remain. Remove from heat and place the mixture in a medium bowl.
Step 3Add the remaining juice to the bowl and stir to combine. Add the evaporated milk and one-fourth cup sugar and stir until dissolved. Place the mixture over an ice bath and stir until chilled and slightly thickened. Remove from the ice bath.
Step 4In a medium bowl, beat the egg whites and salt to soft peaks. With the mixer running, gradually add the remaining one-fourth cup sugar and beat until the meringue is stiff and glossy.
Step 5Add a large spoonful of the egg white mixture to the chilled passion fruit mixture and gently stir until thoroughly combined. Fold in the remaining meringue, one-third at a time, careful not to over-mix.
Step 6Spoon the mousse into individual margarita or stemmed glasses and chill. Sprinkle with the optional reserved passion fruit seeds before serving.