Yield: makes 1 dozen cupcakes
Preheat oven to 350 degrees F. Line muffin tin. Set aside.
In the bowl of stand mixer, beat together sugar and oil. Add mashed bananas. Lightly whisk together egg and yolk, then add into mixture along with vanilla paste. Mix until combined.
In another bowl, whisk together flour, baking soda, and salt. Add flour mixture to mixing bowl in three additions, alternating with buttermilk. Begin and finish with flour mixture. Add walnuts. Mix until just combined.
Divide batter among 12 cupcakes. Bake in preheated oven for 18-20 minutes until cakes are set and done. Let cool in pan for a couple minutes before unmolding and allowing to cool completely on racks.
Whisk together egg whites and sugar over a bain-marie. Whisk constantly until mixture registers 160 degrees F on thermometer.
Transfer mixture to bowl of stand mixer. With a whisk attachment, whisk on medium speed until soft peaks. The mixture should have cooled to a little warmer than room temperature by this point.
With the machine running on med-low speed, add butter a tablespoon a time. Incorporate the fat slowly and ensure the butter is mixed before adding the next tablespoon. Once all the butter is in, add peanut butter and salt and mix on medium high speed. Beat until buttercream is smooth and shiny.
Spread onto cooled cupcakes and sprinkle with chocolate jimmies.
CUPCAKES ADAPTED FROM THE CRAFT OF BAKING