Preheat and Prepare Pans:
Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans with butter or non-stick spray to prevent sticking.

Mix Dry Ingredients:
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

Cream Butter and Sugar:
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy. This should take about 3-5 minutes.

Add Eggs:
Add the eggs, one at a time, beating well after each addition to ensure they are fully incorporated into the batter.

Incorporate Sour Cream and Vanilla:
Mix in the sour cream and vanilla extract, ensuring the batter is smooth and creamy.

Add Mashed Bananas:
Stir in the mashed bananas until fully combined. The bananas add sweetness and moisture to the cake.

Combine Wet and Dry Ingredients:
Gradually add the dry ingredients to the wet mixture, mixing gently until just combined. Avoid overmixing to prevent a dense cake.

Fold in Walnuts:
Gently fold in the chopped walnuts, ensuring even distribution throughout the batter.

Divide and Bake:
Divide the batter evenly between the two prepared cake pans. Smooth the tops with a spatula and bake for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Cool and Assemble:
Allow the cakes to cool completely in the pans. Once cooled, remove the cakes from the pans and level the tops if needed.

Assemble the Cake:
Place one cake layer on a serving plate. Spread a generous layer of vanilla custard over the first layer. Place the second cake layer on top and drizzle the remaining custard over the cake.

Garnish and Serve:
Top with banana slices and additional walnuts for garnish before serving. This adds texture and enhances the flavor.