-
1
Position rack in lower third of oven; preheat oven to 375 degree F.
-
2
Lightly grease a 12 x 15 1/2 x 1/2-inch baking sheet and line pan with foil, leaving a 2-inch overhang at each short end. Grease and flour the foil, tapping excess flour. Pour flour, baking soda, baking powder, and salt into a sifter. Sift onto a sheet of waxed paper ; set aside.
-
3
Crack egg into a small bowl, and whisk briefly. Combine the mashed banana, sour cream, and lemon zest in a small bowl; set aside.
-
4
Place butter in the bowl of a heavy-duty mixer. With a paddle beater, cream the butter on medium speed for 30 to 45 seconds, or until it is smooth and light. Add sugar steadily, scrape the bowl as needed. Continue to cream until mixture is light in color and fluffy in appearance. Pour in the egg and continue to cream for about 1 to 2 minutes. Add half of the flour, stir with a rubber spatula. Add the mashed banana mixture, stirring to blend. Add the remaining flour mixture and stir until smooth.
-
5
Scoop the batter onto five different areas over two-thirds of the prepared baking sheet. With a spatula, spread and coax the batter to cover the two-thirds of the sheet. Extend it to the rest of the sheet in as even a layer as possible. Bake for 8 to 10 minutes, or until the cake is light brown, the sides are beginning to contract from the metal, and the cake springs back when lightly touched in the center. Remove the pan from the oven. Using a knife, release any portion of the cake sticking to the long sides of the pan. Pull up on the foil overhangs, one at a time, to release foil from the pan's edges. Lift up the cake bottom gently and transfer it to a large rack to cool. Place a sheet of foil over the cake in a tent fashion, and let it cool for 30 minutes.
-
6
Slip a baking sheet of the same size under the foil-lined banana sheet cake. Sprinkle 1 tablespoon granulated sugar over the cake's surface. Place two 18-inch strips of foil lengthwise on top of the sugared cake, one overlapping the other; then place a second baking sheet of the same size on top of the foil. Invert the cake on it, and carefully peel the baking foil from the cake.
-
7
Combine the filling ingredients in the bowl of a mixer, with a wire beater whip until soft peak.
-
8
Lift the cake on the two overlapping sheets of foil off the baking sheet so that one of its long sides is parallel to the edge of your counter. Spread the cream evenly over the cake, up to 1 inch on the long end. Begin rolling by flipping the edge of cake nearest you over onto itself. Then, with the aid of the foil, roll the cake lengthwise until you reach the end.
-
9
Sprinkle a light coating of powdered sugar over the cake before serving.
