Directions

1. Preheat oven to 350° F. In a large bowl or the bowl of a stand mixer, whisk together sifted cake flour, vanilla sugar, baking powder, baking soda, and salt until just combined. Add corn oil, mashed bananas, and vanilla. Beat for 1 minute until just combined, then add the buttermilk and egg yolks and beat for 1 additional minute. Set aside to make the meringue.
2. In a separate bowl, with clean beaters, beat egg whites on medium speed. While continuing to beat, add cream of tartar, then 1/2 cup sugar gradually. Beat on high until stiff peaks form. Fold the meringue gently into the cake batter until the streaks just fade.
3. Prepare a 12- x 17-inch jelly roll pan (a large baking sheet with high edges): Butter the edges, then line it with parchment paper and flour it. Pour the batter in and place it into the oven. Bake for about 17 to 20 minutes, or until a toothpick inserted into the center comes out clean. Cool for 10 minutes in the pan on a cooling rack.
4. While the cake cools in the pan, make the rum syrup by combining the remaining 1/4 cup sugar and the rum in a small saucepan over medium heat. Stir and simmer until it thickens, about 2 minutes. Once slightly cooled, flip the cake out onto a cutting board or cooling rack. Stab the cake with a toothpick or bamboo skewer all over, then spoon rum syrup over the surface of the cake. Let the cake cool for one hour.
5. To make the frosting, sift confectioners' sugar into a bowl and set aside. In a large chilled bowl, whip the cream and mascarpone together, gradually adding the confectioners' sugar and vanilla. Whip until firm peaks.
6. To assemble the cake, cut the edges off of it. Cut the cake into thirds and then those thirds in half so that you have six cakes—this will yield two cakes with three layers each. Place one layer on a plate. Prepare a pastry bag with a large star tip and pipe the frosting evenly across the top of the cake. Top the frosting with a single layer of banana slices. Place another layer of cake on top and repeat with the third layer. Pipe frosting into straight lines or stars onto the third layer to decorate. Repeat with the second cake. Refrigerate—this cake tastes best after it rests in the fridge for a few hours, or up to overnight. Serve.