Preheat your oven to 350°F (175°C) and prepare two 8-inch round cake pans by greasing with unsalted butter and dusting with flour.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon until well combined.

In a large mixing bowl, beat the softened butter with the granulated and brown sugars until light and fluffy, about 3-5 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and mashed bananas until smooth.

Gradually mix the dry ingredients into the wet ingredients, alternating with the buttermilk. Fold in the chopped walnuts.

Divide the batter evenly between the prepared cake pans and bake for 25-30 minutes, or until a toothpick comes out clean.

Allow the cakes to cool in the pans for about 10 minutes before transferring to wire racks to cool completely.

In a chilled mixing bowl, whip the heavy whipping cream with powdered sugar and vanilla extract until stiff peaks form.

Once cooled, place one cake layer on a serving plate, spread with whipped cream, and layer sliced bananas. Top with the second cake layer and remaining whipped cream.

Garnish with additional walnuts and banana slices, then chill in the refrigerator for at least 30 minutes before serving.