Sift together the flour, cocoa, baking powder, salt and baking soda.
Combine eggs, water and vanilla and beat until thick and lemon-colored. Add granulated sugar gradually (1 tbsp at a time) and beat thoroughly after each addition.
Slowly sift flour mixture into egg mixture and fold in carefully, but thoroughly.
Grease a 15x10 in jelly-roll pan and line w/waxed paper. Spread batter evenly in pan and bake 30 minutes or till cake tests done. Touch cake lightly w/fingertip. If the dough springs back quickly, the cake is done.
Sprinkle a kitchen towel heavily with confectioner's sugar . Turn cake onto kitchen towel and peel off waxed paper. Trim off any crisp edges. Roll warm cake lengthwise in towel into a roll. Cool on rack, seam side down.
Filling - Cream butter with egg yolk and cocoa. Beat in sugar gradually, alternating w/rum. Keep at room temp for spreading.
Cake Assembly - Unroll cake and spread with filling. Reroll and refrigerate till serving time.