Position an oven rack in the center and preheat oven to 350 degrees. Grease an 8 x 8-inch or 9 x 9-inch square baking pan.
In a large bowl, with an electric mixer, beat the coconut oil and sugar until well blended. Add the eggs and vanilla and beat well. Dissolve the baking soda in the buttermilk and add to the mixture.
Add the mashed bananas and beat until well blended.
Add the flour and stir just until combined.
Pour the batter into the prepared pan, spreading it out evenly.
Bake until a toothpick inserted in the center comes out clean, 45 to 50 minutes.
Remove from the oven and place the baking pan on a wire rack to cool completely.
To make the frosting, place the butter and water in a very small saucepan and heat to boiling. Pour over the sugar. Stir in the peanut butter until well blended and creamy.
Quickly spread the frosting over the cooled cake.
Notes
Serving size: 1 piece (1/16th recipe)WW Points: 9Check your WW Points for this recipe and track it in the WW app. (Must be logged into WW on a smartphone or tablet.)8SmartPoints (Green plan)8SmartPoints (Blue plan)8SmartPoints (Purple plan)5PointsPlus (Old plan)If you don't like peanut butter frosting, feel free to substitute a thin layer of your favorite icing—vanilla, chocolate, cream cheese, and coconut—would all be delicious.