To begin, prepare a 9x9 pan with parchment paper (or spray with non-stick spray) and preheat your oven to 350.
To a large mixing bowl, add your butter, peanut butter, and brown sugar and beat them together with a hand mixer till creamy. Add your banana and mix in well. Add your vanilla and egg and blend in briefly. Add your flour and salt and mix together till you have a thick, consistent dough.
Pour your dough into your pan and spread into a thin, even layer. Bake for 25-28 minutes, or till the top is set and a toothpick inserted comes out clean.
Let cool, then top with frosting.
To a large mixing bowl, add your butter and peanut butter and beat them together with a hand mixer till creamy. Add your powdered sugar and one tbsp of milk, and beat together till smooth. Add more milk as needed.
Spread across the top of your blondies. Cut into 16 pieces, and devour!
Stored in an airtight container in the fridge these last about 5 days.