Banana Cake with Butter-Rum Frosting

I had more very ripe bananas and wanted to make something different than the regular banana bread. So I looked up some recipes and came across this one. I altered it a little and was very happy with the result.  Quick  and delicious!  Enjoy!
For cake:
-1/2 cup unsalted butter, softened
-1 1/3 cups sugar
-1 1/2 tsp baking powder
-1/2 tsp baking soda
-1/4 tsp salt
-1 large egg
-4 very ripe bananas, peeled
-1/4 cup vegetable oil
-1 tsp vanilla extract
-2 cups flour
-1/2 cup walnut topping
For frosting:
-3 Tbs unsalted butter, softened
-3 cups powdered sugar
-1 Tbs rum extract
-1 tsp vanilla extract
-1/4 cup milk
Preheat oven to 350 F.  Coat 9x13 pan (bottom and sides) with baking spray (the one with butter and flour). Set aside.
In your mixer's bowl, beat the butter on medium for 30 seconds.  Add the sugar, baking powder, baking soda and salt.  Mix well. Add the egg, bananas (no need to mash them before, just toss them in), oil and vanilla.  Beat until well combined.  Stir in the flour and walnuts. Mix well and pour in prepared pan, spreading evenly.
Bake in preheated oven for 40-45 minutes, or until toothpick comes out clean.  Remove from oven and place pan onto wire rack.  Cool completely.
While cake is cooling, let's make the frosting.
Beat the butter on medium speed for 30 seconds.  Switch mixer to low and add the powdered sugar, extracts and milk.  Beat until spreading consistency.
Spread that yumminess over cooled cake.  Slice and enjoy!
Original recipe found here.