Preheat oven to 350°F. Grease a 9×11 (or 8.5×11) baking dish.

In a large bowl, cream butter, sugar, eggs, and vanilla until light and fluffy.

In a separate bowl, mix flour, baking powder, baking soda, and salt.

Add ⅓ of the flour mix to the wet ingredients and mix. Then add ⅓ of the mashed banana and mix. Repeat, alternating until all combined.

Pour batter into prepared dish and bake for 32–37 minutes, or until a toothpick comes out clean.

Let cool for 5 minutes in the pan, then flip onto a wire rack to cool completely.