1-3/4 c sugar
1-1/2 c unsifted cake flour
2/3 c cocoa
2 t baking powder
1 t salt
½ t baking soda
½ c oil
7 eggs, separated, at room temperature
¾ c cold water
2 t vanilla
½ t cream of tartar
¼ c sugar
Combine 1-3/4 c sugar, cake flour, cocoa, baking powder, salt, and baking soda in large bowl. Make a well in the mixture and add, in order: oil, egg yolks, water, and vanilla. Beat until smooth.
Beat egg whites and cream of tartar in separate bowl until foamy. Gradually add ¼ c sugar and beat to stiff peaks. Gradually pour chocolate mixture over egg whites, gently folding just until blended. Pour into ungreased tube pan; Bake at 324 for 1 hour and 20 minutes. Prepare mousse and frosting at this time.
Invert cake over heat-proof funnel or bottle; let cool. Loosen cake from pan; invert on serving plate. Slice ¾-inch thick layer from top of cake and set aside. Being careful to leave 1-inch thick walls and base, cut a neat cavity cake. With fork, remove sections of cake between the cuts and spoon mousse into the cavity. Replace top of cake, press gently, and frost. Chill several hours. Refrigerate leftovers.
Mousse Filling
1 envelope unflavored gelatin
2 T cold water
1/3 c water
1/3 c cocoa
2/3 c sugar
1-1/2 c cream
2 t vanilla
Sprinkle gelatin onto 2 T water in small glass dish; set aside to soften.
Bring 1/3 c water to boil; stir in cocoa over low heat until smooth and thickened. Remove from heat; add gelatin; stir until dissolved. Remove from heat, stir in sugar. Cool to room temperature. Whip cream with vanilla to soft peaks. Gradually add chocolate mixture while beating on low speed until well blended; chill 30 minutes.
Chocolate Cream Frosting
¾ c powdered sugar
6 T cocoa
1-1/2 c cream
¾ t vanilla
Combine powdered sugar and cocoa in small bowl Add cream and vanilla; beat until stiff. Cover, chill.