Stir first 3 ingredients in heavy large saucepan over medium-high heat until sugar dissolves and cream comes to simmer. Remove from heat. Add 20 ounces chocolate; whisk until melted and smooth. Whisk in rum and vanilla. Transfer 1 cup warm chocolate mixture to small bowl; whisk remaining 2 ounces chocolate into mixture in bowl and set aside for topping. Cover; let stand at room temperature. Transfer remaining chocolate mixture to large bowl to use as filling; chill until cold and thick, at least 6 hours and up to 1 day.
Stir water and sugar in small saucepan over low heat just until sugar dissolves. Remove from heat; mix in rum. Cover and let stand up to 1 day.
You can find complete recipes of this MOCHA LAYER CAKE WITH CHOCOLATE-RUM CREAM FILLING in epicurious.com