Preheat oven to 350 degrees F. Line a 12X17-inch jelly roll pan with parchment paper. Place eggs and sugar into mixer bowl and whisk on high speed until fluffy and pale, almost white in color. Combine the flour and baking powder in small bowl. Using sifter or mesh strainer, sift flour into egg mixture in increments; use a spoon to fold the flour in very carefully.
Spread the cake batter onto the prepared baking sheet and bake in preheated oven for 13 to 15 minutes until top turns golden brown. Remove from oven and immediately loosen edges of the cake away from the pan using a knife. Then place a clean kitchen towel onto the cake and gently roll up the cake. Place into cooling rack to cool completely.
Orange Prune Filling:
While cake is cooling, prepare the prune filling. Finely dice the prunes and place in medium sauce pot along with the espresso, orange juice and rum. Return to stove top and cook over medium heat, for 15 to 17 minutes until mixture is thick and most of the liquid has evaporated. Remove from heat and place into blender or use stick blender to puree the filling. Let the filling cool before using on cake.
To prepare Chantilly crème, place cold whipping cream, sugar and rum into mixer bowl. Whisk on high speed until stick peaks form. Place into the refrigerator if not using immediately.
Assembling the cake:
To assemble cake, unroll the cooled sponge cake and remove the parchment paper and towel. Spread with the prune filling, spreading it evenly across the cake. Follow with half of the Chantilly crème. Gently roll the cake into jelly roll and place onto cake platter. Spoon the remaining Chantilly crème on top and use cake spatula to spread the crème evenly on all sides. To garnish cake, use chocolate, cocoa powder, sliced nuts such as almonds or fresh berries. Place cake in refrigerator overnight (preferably).
You can add optional additional filling: simply whisk together butter and dulce de leche until light and fluffy. Spread onto sponge cake before adding other fillings.