Preheat oven to 350°F. Grease and flour 2 9-inch round cake pans; set aside.
In a medium bowl stir together the mashed banana and the lemon juice; set aside. In another medium bowl stir together the flour, baking soda, and salt; set aside.
In a large mixing bowl beat the butter with an electric mixer on medium to high speed for 30 seconds. Add the granulated sugar and beat until light and fluffy, about 2 minutes. Beat in eggs, one at a time, on medium speed until combined. Beat in 2 teaspoons vanilla. Add flour mixture and buttermilk alternately to the beaten mixture, beating on low speed after each addition until combined. Fold in banana mixture. Divide batter between prepared pans. Bake for 35 minutes or until a toothpick inserted near the centers comes out clean. Remove and cool in pan on a wire rack for 10 minutes. Remove from pans and cool completely on the wire racks.
Place one cake on a plate, top side down. Spread with strawberry jam. Add remaining cake layer, top side down. Chill while preparing the ganache.
Place chopped chocolate in a medium heat-proof bowl; set aside. In a small saucepan heat the cream just to simmering. Pour over chocolate in bowl. Allow to stand 5 minutes without stirring. Stir until smooth. Stir in remaining 1 teaspoon vanilla. If necessary, let stand until slightly thickened.
Spoon ganache over top of stacked cake layers, spreading right to the edges so it will drip down. Chill for 30 minutes or until chocolate is set.
Gently place stencil on top of cake. Sift powdered sugar over the top of cake and gently lift the stencil.
Trace the bottom of the cake pan on a piece of parchment paper. Cut the circle out with scissors. Carefully draw the design on the paper, and using a mat and sharp knife, cut out the design. Or, you can use a shaped scrapbook punch.