Preheat the oven to 350 degrees. Prepare baking dish with butter and flour or nonstick spray.

Melt butter and set aside to cool.

In a large bowl, mash bananas with fork or hand held mixer. Add butter, sugar, eggs and vanilla and mix well. Add in flour and stir until ingredients are incorporated.

Bake for 18-25 minutes. You want the top to be a light brown and an inserted toothpick comes out clean. Cake should have risen in the middle and spring back if you press gently with your finger.