Chocolate and bananas are a typically American combination, but they have found their way into this very Viennese dessert. In order to mold the filling, you’ll need to create an aluminum foil collar to wrap around the cake—a simple task.
½cup sifted cake flour
3tablespoonsDutch-processed cocoa powder
pinch of salt
3 large eggs, at room temperature
Vegetable oil cooking spray, for the foil collar
6ouncesbittersweet chocolate, finely chopped
3ripe bananas, peeled and cut in half lengthwise
2ouncesbittersweet chocolate, finely chopped
To make the cake: Position a rack in the center of the oven and heat to 350°F. Butter an 11½ X 8-inch baking pan and line the bottom with parchment or wax paper. Dust the inside of the pan with flour to coat the sides and tap out the excess.
Sift the cake flour, cocoa, and salt together. Whisk the eggs and sugar in the bowl of a heavy-duty standing mixer. Place over a medium saucepan of simmering water (the water should not touch the bottom of the bowl) and whisk constantly until the eggs are hot to the touch and the sugar is dissolved (rub a bit between your fingers to be sure). Attach to the mixer and fit with the whisk. Beat on high speed until the eggs have tripled in volume and are light and fluffy, about 3 minutes. (If you use a handheld electric mixer, allow 5 minutes to reach this consistency.)
Meanwhile, melt the butter in a small saucepan over low heat. Add the milk and heat until hot to the touch. Pour into a medium bowl and stir in the vanilla.
In two additions, sift the cocoa mixture over the eggs and fold in, using a large balloon whisk or rubber spatula. Transfer a large dollop of the batter into the milk mixture and whisk to combine. Pour into the bowl of batter and fold in. Spread evenly in the pan.
Bake until the top springs back when pressed in the center, about 20 minutes. Cool on a wire rack for 5 minutes. Invert and unmold onto the rack, and remove the paper. Cool completely.
To make the filling: Bring the cream to a simmer in a small saucepan over medium heat. Remove from the heat and add the chocolate and rum. Let stand for 3 minutes, then whisk until smooth. Transfer to a medium bowl set in a larger bowl of ice water. Let stand, stirring occasionally, until cooled but not set. With the bowl still in the water, whisk until the mixture is thick and spreadable. Do not overbeat.
Place 2 of the bananas on the cake, cut sides down, trimming and piecing the remaining banana to fit, discarding (or snacking on) any leftovers. Spoon the chocolate filling over the bananas, and spread evenly to cover the bananas, almost to the edge of the cake.
Fold a 40-inch-long piece of aluminum foil in half lengthwise to make a 6-inch-tall strip. Spray the foil with the oil. Oiled side in, wrap the foil strip around the perimeter of the cake to make a collar, and tape it in place. Crimp the foil so it makes a tight fit. Spread the chocolate filling to touch the collar. Place the cake on a serving plate. Refrigerate until the filling is chilled and set, about 2 hours.
To make the glaze: Bring the cream to a simmer in a small saucepan over medium heat. Add the chocolate and corn syrup and whisk until smooth. Cool slightly.
Remove the foil collar. Pour all of the glaze on top of the filling. Using an offset metal spatula, immediately spread in a thin layer just to the edge (don’t worry if some glaze drips down the sides). Refrigerate until the glaze sets, about 30 minutes. To serve, use a hot, wet knife to slice into 8 pieces. Serve chilled.