Bring 1/2 cup water to a boil in a small saucepan. Add cocoa and espresso, stirring until smooth. Remove pan from heat. Add chocolates; gently stir until mixture is smooth. Stir in brandy and vanilla. Pour chocolate mixture into a large bowl. Let stand 10 minutes; stir occasionally.
Combine eggs, egg whites, sugar, and salt in the top of a double boiler, stirring with a whisk. Cook over simmering water until a thermometer reaches 115° (about 2 minutes), stirring constantly with a whisk. Place egg mixture in a medium bowl; beat with a mixer at high speed until ribbony, soft peaks form (about 5 minutes).
Gently stir one-third of egg mixture into chocolate mixture; gently fold in remaining egg mixture. Gently fold in whipped topping. Spoon batter into an 8-inch springform pan coated with baking spray, spreading evenly. Bake at 350° for 27 minutes or until almost set (center will not be firm but will set as it chills). Cool to room temperature on a wire rack. Cover and chill at least 8 hours or overnight.
OR USE WHIPPED CREAM: If you substitute an equal amount of whipped cream for the whipped topping, you'll add 62 calories, 7g fat, and 2g sat fat to each serving. You will also need to bake the mousse about 5 minutes longer.