Preheat oven to 350F. Grease and flour a 9" x 5" loaf pan (I use homemade cake release).
In a small bowl, whisk together flour, baking soda, and salt. Set aside.*
In a medium bowl, whisk together peanut butter, mashed bananas, oil, sugar, egg, and vanilla.
Add dry ingredients to wet and mix until combined.
Pour half of batter into prepared pan, add cocoa powder to remaining batter and mix well. Dollop onto peanut butter batter and marble with a skewer.
Bake for approx. 50 mins or until a cake tester comes out mostly clean.
Cool in pan on wire rack for 10mins, turn out and cool completely on wire rack.
Peanut Butter Glaze:
In a small bowl, whip together peanut butter, powdered sugar, and 3Tbsp cream (I did this by hand). Add more cream until it reaches a desired consistency.
Spread over cooled loaf. Dust with cocoa powder if desired.
Notes
* For one-bowl method skip this set and add dry directly to wet.