For 15-16 cupcakes:
6 Tbsp butter, room temp
1 1/4 c sugar
1 large very ripe banana
3 eggs
1/2 c buttermilk - I use Saco powdered buttermilk, which would be 2 Tbsp powder and 1/2c water
1 tsp vanilla
1 1/4 c flour ( + 1 or 2 Tbsp if your batter seems thin)
2 tsp. baking powder
pinch of salt
PB Frosting:
4 Tbsp butter, room temp
4 oz cream cheese, room temp
2 - 3 c powdered sugar
1/2 - 3/4 c smooth peanut butter
Preheat the oven to 350, and line 15-16 baking cups with liners.  Give each liner a spritz of Pam to ensure that the cakes don't stick.  
In the bowl of a mixer, cream the butter and sugar until light and fluffy.  Add the banana and mix to break up.  Add the eggs one at a time, scraping the sugar up in between eggs.  Add the buttermilk and vanilla and combine.  Mix in the baking powder and salt, then add the flour in 2 additions, scraping the bottom and sides of the bowl in between.  If the batter seems thin, add 1 or 2 Tbsp flour.  Fill the pans 3/4 full and bake for 18 - 22 min.  They are done when no batter sticks to a toothpick inserted in the middle.  Let cool for a few minutes in the pan, then move to a wire rack to finish cooling.
For the frosting:  in the bowl of a mixer, beat the butter until creamy.  Add the cream cheese and combine.  On low, beat in 2 cups powdered sugar.  Taste and check consistency.  If not sweet enough or if not thick enough, beat in another 1/2 cup or more of powdered sugar.  Finally, mix in the peanut butter and taste.  You can always add more PB :)    Either spread or pipe lots of frosting onto each cupcake when cool.  I store the cakes in a container on the counter and the frosting in the fridge, then frost each cupcake as I need to.
 
