Cream the butter for 2-3 minutes with a hand mixer in a large bowl.
Mix the sour cream, eggs, and vanilla into the creamed butter.
Stir the mashed bananas and cake mix into the liquid ingredients. Stir until just combined.
Spread the batter in a greased and parchment paper lined 15x10x1 inch jelly roll pan and bake at 350 degrees for 20-25 minutes. When a toothpick comes out clean, the cake is done.
Spread frosting or other desired frosting toppings.
Cream Cheese Frosting
Cream the cream cheese, butter, and vanilla together
Slowly add the powdered sugar to the cream cheese mixture. When banana bars are cooled spread icing over top of the cake.
Notes
If you prefer homemade instead of boxed cake mix, you can substitute it with a homemade cake mix. Use 1 3/4 cups of all-purpose flour, 1 1/2 teaspoons of baking powder, 1/2 teaspoon of baking soda, 1/4 teaspoon of salt, and 3/4 cup of granulated sugar.