Preheat oven to 350 degrees (F). Lightly grease a doughnut pan; set aside.
In a large bowl whisk together the dry ingredients. In a separate bowl whisk together the mashed bananas, melted butter, and egg. Gently fold the wet mixture into the dry mixture - don't over mix!
Spoon mixture into prepared doughnut pan and bake for 10-11 minutes, or until the doughnuts spring back when lightly pressed. Allow doughnuts to cool a few minutes in the pan before transferring to a wire wrack to cool completely.
For the cream cheese frosting:
In the bowl of a stand-mixer fitted with the paddle attachment, or in a large bowl using an electric handheld mixer, beat the cream cheese, butter, and vanilla extract on medium-high speed until completely smooth; about 2 minutes. Reduce the speed to low and gradually add the sifted confectioners' sugar. Add in the milk. Once the milk is combined beat frosting on high-speed for 1-2 minutes.
Spread frosting on cooled donuts and sprinkle with chopped walnuts.
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