Mash the bananas in a large mixing bowl. (I like to use my KitchenAid mixer and paddle attachment for this). Mix in the vanilla and eggs.
In a separate bowl, whisk together the flour, baking soda, baking powder, salt, sugar, and spices. Mix into the banana mixture. Add the oil and milk and mix until fully combined.
Pour batter into greased sheet pan and bake for 25 minutes. Let cool before frosting.
FOR THE FROSTING
Cream together the butter, cream cheese, and vanilla using a handheld or stand mixer fitted with a paddle or whisk attachment. Mix in the powdered sugar 1 cup at a time until fully combined. Add the evaporated milk as needed for desired consistency.
PUTTING IT TOGETHER
Once the banana cake is cooled completely, spread the cream cheese frosting evenly across the top. If desired, sprinkle with chopped walnuts.