Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line with parchment paper.
In a medium bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.
In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy, about 3 to 4 minutes.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
Mix in the mashed bananas followed by the sour cream until fully combined.
Gradually add dry ingredients to wet mixture, mixing just until combined. Fold in toasted walnuts.
Divide the batter evenly between prepared pans and smooth the tops.
Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove cakes from oven; cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
To make the whipped cream filling and frosting, use a chilled bowl and beat heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
Place one cake layer on a serving plate or cake stand. Spread a generous layer of whipped cream over the top.
Place the second cake layer on top and frost the top and sides with remaining whipped cream using an offset spatula.
Garnish with finely chopped walnuts and banana slices just before serving.
Slice and serve chilled or at room temperature.