Beat butter at medium speed with an electric mixer until fluffy; gradually add sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition. Add rum; beat until blended.
Combine flour and next 5 ingredients; add to sugar mixture alternately with hot water, beginning and ending with flour mixture. Beat at low speed until blended after each addition; stir in flavorings.
Pour batter into 2 greased and floured 9-inch round cakepans.
Bake at 350° for 27 minutes (cake will not test quite done). Cool in pans on a wire rack 10 minutes; remove from pans, and cool completely on wire racks.
Spread 1 cup Mousse Frosting between layers. Frost top and sides of cake with remaining frosting. Garnish, if desired. Cover and chill 8 hours. Store in refrigerator.
The cake slices well after chilling 1 hour, but plan to refrigerate it overnight before serving to allow the flavors to mellow.