Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set this aside for later.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the eggs one at a time, and then add the vanilla extract.
Gradually mix in the mashed bananas, and then alternate adding the dry ingredients with the buttermilk, mixing until everything is just combined.
Stir in the chopped walnuts for that lovely crunch.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.
For the frosting, whip together the heavy cream, powdered sugar, and vanilla extract until soft peaks form, and then beat in the softened cream cheese until it's smooth and creamy.
Once the cake has cooled, spread cream cheese frosting over the top of one layer. Place the second layer on top, add more frosting, then place the third layer and spread frosting on top and sides. Lastly, sprinkle with chopped walnuts and sliced bananas for garnish.
Slice, serve, and enjoy!