Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt.
In another bowl, cream the softened butter until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and mashed bananas.
Gradually add the dry ingredients to the wet mixture, alternating with buttermilk. Mix until just combined. If using, fold in the chopped walnuts.
Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring to wire racks to cool completely.
For the frosting, beat the softened butter until creamy. Gradually add the powdered sugar, mixing well. Add the vanilla extract and heavy cream, beating until smooth and fluffy.
Once the cakes are completely cool, spread a layer of frosting on top of one cake layer, place the second layer on top, and frost the top and sides of the cake. Garnish with sliced bananas.