Preheat your oven to 350°F (175°C) and prepare a 9-inch round cake pan by greasing it with butter and dusting it with flour.

In a medium mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.

In a large bowl, cream softened unsalted butter and granulated sugar together until light and fluffy, about 3-4 minutes. Add eggs one at a time, mixing well.

Gently stir in mashed ripe bananas and vanilla extract into the butter and sugar mixture.

Gradually add the dry ingredients to the wet ingredients, alternating with buttermilk, starting and ending with the flour mixture.

Fold in the chopped walnuts until evenly distributed.

Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick comes out clean.

After baking, cool the cake in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Whip heavy cream in a chilled bowl, gradually adding powdered sugar and vanilla until soft peaks form.

Once the cake is cooled, frost the top or slice it horizontally to create layers, spreading the frosting generously.

Garnish with additional chopped walnuts and chill in the refrigerator for at least 1 hour before serving.