Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

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, followed by the mashed bananas and vanilla extract. Mix until well combined.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, alternating with the sour cream. Mix until just combined.

Fold in the chopped walnuts gently into the batter.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean.

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

While the cake cools, whip the heavy cream in a bowl until soft peaks form. Gradually add the powdered sugar and vanilla extract, continuing to whip until you achieve stiff peaks.

Once the cake is fully cooled, slice it in half horizontally and spread a layer of whipped cream over the bottom half.

Place the other half on top and frost the top and sides of the cake with the remaining whipped cream.

Top the cake with extra chopped walnuts if desired. Chill in the refrigerator before serving.