1. Make filling: Soften gelatin in water in small saucepan; stir in coffee; heat slowly, stirring constantly, just until gelatin dissolves; cool.
2. Beat butter or margarine with sugar until fluffy in large bowl of electric mixer at high speed; add eggs, 1 at a time, beating about 3 minutes after each addition. Stir in cocoa and vanilla; slowly add cooled gelatin mixture; fold in marshmallow cream.
3. Pour into a wax-paper-lined 8-inch layer cake pan; chill 2 hours, or until filling is firm.
4. Make cake: Measure flour, cocoa and baking powder into sifter.
5. Beat butter or margarine with sugar until fluffy in large bowl of electric mixer at high speed; beat in eggs, 1 at a time. Sift in dry ingredients, stirring just until well blended with wire whip- stir in vanilla and nuts. Pour into a greased 9-inch layer cake pan, spreading evenly.
6. Bake in slow oven (300°) 35 minutes, o!r until cake begins to pull away from side of pan. Cool in pan on wire rack 10 minutes; remove from pan; cool completely.
7. Split cake crosswise with sharp thin-bladed knife to make 2 thin layers; place 1 on serving plate.
8. Run a spatula around edge of molded filling to loosen from panturn out onto cake layer on plate; peel off wax paper. Top with second layer. Chill while making frosting.
9. Make frosting: Heat cocoa, butter or margarine and rum in small saucepan until butter is melted; remove from heat. Stir in 10X sugar and vanilla; beat until smooth. (Frosting will be thin.) Pour immediately over top of filled cake, spreading to edge with spatula.
10. Chill until ready to serve. (Cake will hold well if made a day ahead.) Slice into thin wedges with a long, sharp slicing knife.