Whisk flour, baking soda, baking powder, and salt in a bowl.
Beat butter or oil with sugar until creamy and light.
Mix in eggs, mashed bananas, sour cream, and vanilla until smooth.
Stir dry ingredients into wet ingredients until just combined.
Pour into greased 9×13-inch pan and bake at 350°F (175°C) for 30–35 minutes.
Let the cake cool completely before frosting.
Beat cream cheese and butter until smooth, add sugar and vanilla, and whip until fluffy.
Spread frosting evenly, top with nuts, slice, and enjoy.
