Preheat the oven to 350℉ and grease a 9X13-inch glass baking dish with non-stick cooking spray.
In a large bowl using a handheld electric mixer, cream together the butter and sugar, add the vanilla, eggs, and mashed bananas and beat in medium speed until all is combined.
Whisk together the flour, baking powder, and baking soda in a medium-size bowl. Add this mixture to the wet ingredients and mix on low speed until just combined. Lastly, add the milk and beat just until incorporated.
Bake the cake in the preheated oven for 35-40 minutes or until a toothpick inserted into the center comes out clean. Remove from oven a let cool completely before frosting. Enjoy!
For the Frosting:
I a medium bowl, beat together the butter and peanut butter using a handheld electric mixer on medium speed. Gradually add the icing sugar and mix on low speed to combine. Lastly, add the milk and beat until smooth and creamy. Spread the icing over top of the completely cooled cake in an even layer.