For the Bottom Layer:

1. Preheat oven to 325°; spray a 9-inch springform pan with non stick cooking spray.
2. Melt butter, chocolate and espresso powder in a large microwave safe bowl; 30 seconds at a time until melted and creamy when stirred.
3. Whisk in vanilla and egg yolks; set aside.
4. Mix using a whisk attachment the egg whites and salt until frothy, about 30 seconds.
5. Add half of the brown sugar and beat until combined, about 15 seconds.
6. Add remaining brown sugar and beat at high speed until soft peaks form when whisk is lifted, about 1 minute longer, scraping down the sides halfway through.
7. Using whisk, fold one third of beaten egg whites into chocolate mixture.
8. Using rubber spatula, fold in remaining egg whites until no white streaks remain.
9. Carefully transfer batter to prepared spring form pan, gently smoothing top with offset spatula.
10. Bake until cake has risen, is firm around edges, and center has just set but is still soft, 13 to 18 minutes.
11. Transfer cake to wire rack to cool completely, about 1 hour. (Cake will collapse as it cools.)
12. DO NOT REMOVE CAKE FROM PAN.

For the Middle Layer:

1. Combine cocoa powder and hot water in a small bowl; set aside.
2. Melt chocolate in a large microwave safe bowl; heat until melted and smooth.
3. Using the whisk attachment beat together the whip cream, sugar and salt on medium speed until mixture begins to thicken, about 30 seconds.
4. Increase speed to high and whip until soft peaks form when whisk is lifted, 15 to 60 seconds.
5. Whisk cocoa powder mixture into melted chocolate until smooth.
6. Using whisk, fold one-third of whipped cream into chocolate mixture to lighten.
7. Using a rubber spatula, fold in remaining whipped cream until no white streaks remain.
8. Spoon mousse into springform pan over cooled cake and gently tap pan on counter 3 times to remove any large air bubbles; gently smooth top with offset spatula.
9. Wipe inside edge of pan with damp cloth to remove any drips.
10. Refrigerate cake at least 15 minutes while preparing top layer.

For the Top Layer:

1. In a small bowl, sprinkle gelatin over water; let stand at least 5 minutes.
2. Place white chocolate in a medium bowl.
3. Heat 1/2 cup cream in a small microwaved safe bowl; add gelatin mixture and stir until fully dissolved.
4. Pour cream mixture over white chocolate and whisk until chocolate is melted and mixture is smooth, about 30 seconds.
5. Cool to room temperature, stirring occasionally, 5 to 6 minutes.
6. Using the whisk attachment whip remaining cream at medium speed until it begins to thicken, about 30 seconds.
7. Increase speed to high and whip until soft peaks form when whisk is lifted, 15 to 60 seconds. Using whisk, fold one third of the whipped cream into white chocolate mixture to lighten.
8. Using rubber spatula, fold remaining whipped cream into white chocolate mixture until no white streaks remain.
9. Spoon white chocolate mousse into pan over middle layer.
10. Smooth top with offset spatula.
11. Return cake to refrigerator and chill until set, at least 2-1/2 hours.