Preheat the oven to 350 degrees. Spray a 9x9 baking dish (or 7x10, my jam) with non-stick cooking spray.
Whisk the egg in a small dish. Whisk bananas, yogurt, oil, vanilla, maple syrup, and egg until well combined.
Stir in flour, baking soda, and salt. Pour into prepared dish.
Bake for 35-45 minutes, until a toothpick can be inserted and comes out clean.
Allow to cool 20 minutes. While it cools, mix the frosting.
Combine softened butter and cream cheese with a handmixer until smooth.
Add vanilla and mix again.
Slowly blend in pinch of salt and powdered sugar, ¼ cup at a time. Add milk, ½ tablespoon at a time until the desired consistency is acquired.
Frost the cake when the cake is not entirely cool to lock in the moisture. Top with ground walnuts
Serve immediately. Store leftovers in the refrigerator.