Preheat oven to 350°F and line some cookie sheets with parchment paper
Place cake mix, eggs and vanilla in a large mixing bowl.
Add 1/2 cup of mashed bananas and measure how much extra you have.
Take your one cup of water and subtract how much extra mashed banana you have from that one cup. For example, if you have 1/4 cup of extra mashed banana, use 3/4 cups of water.
Add your extra mashed banana and water to the cake mix.
Stir until everything is smooth.
Using a cookie scoop or an ice cream scoop so you get even amounts, place rounded dollops of cake batter on your prepared cookie sheets, leaving plenty of room for them to spread. I had to bake mine in 2 batches, as I got about 20 rounds and could only fit 12 in the oven at a time.
Bake for about 10-12 minutes, or until golden on the bottom and the center of the round seems firm and does not collapse if you gently touch it.
Let whoopie pie rounds cool completely before adding the filling. You can store them on pieces of parchment paper sprinkled generously with powdered sugar, but note they will stick to each other if you pile them one on top of the other!