Stir chocolate, 1/3 cup cream, 1/3 cup water and corn syrup in heavy medium sauce-pan over medium-low heat until chocolate melts and mixture is smooth. Let cool until just lukewarm, stirring occasionally, about 20 minutes. Whisk in rum, then mascarpone. Beat 1 cup cream in medium bowl until stiff peaks form. Fold cream into chocolate mixture in 2 additions.
Peel and thinly slice 2 bananas on diagonal. Arrange 3 slices in each of eight 8- to 10-ounce goblets or coupes. Spoon mousse atop bananas, dividing equally. Cover and refrigerate at least 4 hours. (Can be made 1 day ahead; keep chilled.)
Beat remaining 1 cup of cream in medium bowl until firm peaks form. Spoon or pipe cream decoratively atop each mousse. Peel and slice remaining banana. Garnish each mousse with banana slices.
