Make sure that your bananas are soft and ripe to give the cake the best flavor and texture. This recipe forms the base of our Banana Split Bombe dessert.
Preheat oven to 350 degrees. Butter a 9-inch round cake pan. Line with parchment cut to fit, and butter and flour parchment. Sift flour, baking soda, baking powder, salt, cinnamon, and a pinch of cloves into a medium bowl. Whisk mashed banana, buttermilk, and vanilla in a small bowl.
Beat butter and sugar with a mixer on medium speed until smooth. Add egg, and beat until well combined. Reduce speed to low, and add flour mixture in 2 additions, alternating with banana mixture. Add chopped banana, and beat until well combined. Pour batter into pan, and use a spatula to spread into an even layer.
Bake until golden brown and a tester inserted in the center comes out clean, about 20 minutes. Let cake cool in pan on a wire rack for 10 minutes. Run a knife around edge of pan, and invert pan onto rack. Remove parchment, and flip cake so top side faces up. Let cool completely on wire rack.