For the sponge cake, preheat oven to 375 degrees F. Grease the sides and bottom of a 9-inch round springform pan with 2-1/2- to 3-inch sides, using some of the softened butter. Line bottom of pan with parchment paper. Butter the paper and flour the pan; set aside.
In a small bowl, stir together 1 cup flour, 1/4 cup cocoa powder, baking powder, and baking soda; set aside. In a large heatproof bowl, whisk together 4 eggs and 3/4 cup sugar. Add 1 to 2 inches of water to a large saucepan or 4-quart Dutch oven; bring to boiling. Reduce heat. Set bowl with eggs and sugar over simmering water in pan, making sure the bowl does not touch the water. Heat and whisk the egg mixture constantly for 10 to 15 minutes or until egg mixture is thickened and nearly double in volume. Or beat the egg mixture with an electric mixer on medium speed for 5 minutes or until egg mixture is thickened and nearly double in volume. Carefully remove the bowl from over the heat. With a large rubber spatula, gently fold the flour mixture into the egg mixture. Spread batter into the prepared springform pan.
Bake 15 to 18 minutes or until top of cake springs back when lightly touched and edges of cake just begin to pull away from sides of pan. Cool in pan on a wire rack for 10 minutes. Loosen and remove sides of pan; cool cake completely.
For the zabaglione filling, in a small saucepan, melt the chocolate over low heat, stirring until smooth; set aside. Pour the cold water into a small heatproof custard cup or glass measure; sprinkle gelatin over the cold water. Let stand 5 minutes or until gelatin softens. Add 1 to 2 inches of water to a small saucepan; bring water to a simmer. Place custard cup or glass measure in simmering water (water should come about halfway up side of cup or measure). Stir until gelatin dissolves; set aside.
In a medium heatproof bowl, whisk 4 egg yolks, 1/4 cup sugar, and Marsala until smooth. Add 1 to 2 inches of water to a medium to large saucepan; bring to boiling. Reduce heat. Set bowl with yolk mixture over simmering water in pan, making sure the bowl does not touch the water. Heat and whisk mixture for 5 to 10 minutes or until mixture has thickened and an instant-read thermometer inserted in the mixture registers 145 degrees F for 3 minutes or 160 degrees F, scraping bowl occasionally. (Watch temperature carefully at end to avoid lumps.) Or beat the egg yolk mixture with an electric mixer on medium speed for 10 minutes or until mixture has thickened and an instant-read thermometer inserted in the mixture registers 145 degrees F for 3 minutes or 160 degrees F, scraping bowl occasionally. Carefully remove from heat.
Add the gelatin mixture to the egg yolk mixture and stir until combined. With a rubber spatula, gently fold the melted chocolate into the egg mixture. Let the mixture stand about 10 minutes or until cool.
Meanwhile, in a large bowl, beat 3/4 cup whipping cream with an electric mixer or whisk until soft peaks form (tips curl). With a rubber spatula, gently fold the whipped cream into the chocolate mixture.
To assemble the cake, remove cake from pan bottom and peel off parchment paper. With a long, thin serrated knife, slice cake horizontally into two equal-size layers. Set the bottom layer, cut side up, back onto the springform pan bottom. Drain the cherries, reserving the syrup. Brush 3 to 4 tablespoons of the cherry syrup over the bottom cake layer. Arrange drained cherries on cake layer. Spoon zabaglione filling over the cherries, spreading evenly. Place top cake layer, cut side up, over zabaglione filling. Brush 3 to 4 tablespoons of the remaining cherry syrup over cake. (Discard remaining syrup.) Replace sides of springform pan. Cover cake and chill at least 4 hours or up to 24 hours.
To serve, in a large bowl, beat 1 cup whipping cream, 1 tablespoon sugar, and vanilla with an electric mixer or whisk until soft peaks form. Remove the sides of the springform pan. Place cake on a serving plate. Spread whipped cream mixture over cake. Sprinkle with additional cocoa powder. Top with additional cherries, if desired. Cut the cake into wedges with a thin, serrated knife. Makes 12 servings