Melt the butter and pour it into a large mixing bowl. Add the sugar and mix well. Add the eggs, vanilla and mashed bananas, mixing it all together well. Set aside while you mix the other ingredients.
Add the baking soda to the sour cream in a small bowl and mix to combine.
Combine the flour, baking powder and salt in a separate bowl. Add the flour mixture to the butter mixture alternatively with the sour cream. Mix until all of the ingredients are incorporated together into a batter. Pour the batter into the pan and spread to even it out in the pan.
If you want you can slice a banana and add the slices to the top of the cake batter just before baking. It makes a very pretty presentation for a plain cake.
Bake for 50-60 minutes or until the cake is done. Test for doneness by inserting a toothpick into the cake. If it comes out sticky, the cake needs to continue baking. If it's clean, the cake is done.
Cool to serve. Cut into squares.
Makes 12 to 16 servings.
Notes
Variation: Add 3/4 cup chopped walnuts to the banana bread recipe. It adds a nice crunch.