Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
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, then add the vanilla extract.
Gradually mix in the mashed bananas, then alternate adding the dry ingredients with the buttermilk. Mix until just combined.
Stir in the chopped walnuts.
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.
Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.
For the frosting, whip the heavy cream, powdered sugar, and vanilla extract until soft peaks form.
Beat in the softened cream cheese until smooth.
Once the cake has cooled, spread the cream cheese frosting over the top and sprinkle with additional chopped walnuts for garnish.