Heat oven to 350°F and grease two 8-inch round pans.
Whisk flour, baking soda, and salt; set aside.
Beat butter and sugar until light and fluffy.
Add eggs one at a time, then mix in vanilla, mashed bananas, and sour cream.
Gradually fold in dry ingredients; stir just until combined.
Fold in chopped walnuts and white chocolate chips.
Divide batter evenly between pans and bake 30–35 minutes until golden.
Let layers cool completely before frosting.
Spread half the custard between layers, top with remaining custard.
Add sliced bananas and walnuts; chill before serving.
