Sprinkle gelatin over 1/3 cup water in a small bowl. Let soften 5 minutes.
Bring 1 1/4 cups cream to a boil in a small saucepan over medium-high heat. Add gelatin mixture; stir until dissolved.
Put chocolate into the bowl of a food processor. With machine running, carefully pour in hot cream mixture in a slow steady stream, and process until smooth. Transfer to a medium bowl, and cover with plastic wrap. Refrigerate until thick enough to hold ribbons, about 30 minutes. Stir until smooth.
Whisk remaining 2 1/2 cups cream to nearly stiff peaks. Fold into chocolate mixture. Transfer 2 1/2 cups of the mousse to another medium bowl, and stir in zest. Mousses can be refrigerated, covered, up to 3 hours. Stir until smooth before using.