For Caramel Mousse: Heat 3/4 cup whipping cream in heavy small saucepan to lukewarm. Remove from heat. Mix sugar with 5 tablespoons water in heavy saucepan over medium heat until sugar dissolves. Increase heat and boil without stirring until sugar turns deep golden brown, occasionally swirling pan and washing down sides with wet pastry brush, about 8 minutes. Add warm whipping cream (mixture will bubble up) and cook over low heat until mixture is smooth, stirring constantly. Mix in 3 tablespoons unsalted butter. Pour caramel into small bowl. Cover and refrigerate until cold, about 2 hours.
Beat 1 1/4 cups chilled whipping cream and 1 teaspoon vanilla extract to soft peaks in large bowl. Reserve 3 tablespoons caramel for topping. Fold remaining caramel into whipped cream. Cover and refrigerate.
For Chocolate Mousse: Melt 5 ounces semisweet chocolate in large bowl set over saucepan of simmering water, stirring until chocolate is smooth. Cool to lukewarm, stirring occasionally. Using electric mixer, beat chilled whipping cream, powdered sugar, unsweetened cocoa powder, dark rum and vanilla extract in another large bowl until stiff peaks form. Whisk 3/4 cup of whipped cream mixture thoroughly into melted chocolate. Fold remaining cream mixture into chocolate.
Spoon half of chocolate mousse in bottom of 8 large (10-to 12-ounce) clear stemmed glasses; smooth surface of each. Spoon half of caramel mousse over, spreading to edge of each glass. Repeat chocolate and caramel mousse layers. Drizzle some of 3 tablespoons reserved caramel over each. Cover and refrigerate at least 4 hours.
