In a small bowl combine warm milk (between 100-110 F), sugar, and yeast and let sit 5 minutes or until foamy.
In a stand mixer with dough hook attachment (or large bowl if mixing by hand) combine mashed banana, salt, butter, egg, and 4 cups of flour.
Pour foamy yeast mixture in and mix with a dough hook on low speed for 4-5 minutes until smooth and elastic. If mixing without a mixer, simply mix with a wooden spoon until you can’t mix anymore and then begin kneading by hand 6-8 minutes.Add another ½ cup (up to one cup if needed) of flour to make dough consistency thick and slightly sticky.
Oil a large bowl, place dough in it and cover with plastic wrap. Let rise at room temperature for one hour or until doubled in size.
For the filling:
In a medium sized bowl combine mashed banana, vanilla extract, brown sugar, cinnamon, and flour.
On a floured work surface with rolling pin, roll dough into roughly a 24 inch by 12-14 inch rectangle.
Spread softened butter over the dough with a spoon or pastry brush. Spread banana filling evenly over butter, then sprinkle chopped walnuts on top.
Carefully roll dough lengthwise into a long roll. Mark the dough into 12 equal portions with a knife.Use a piece of clean string or dental floss and slide the string underneath the roll. Pick up both ends of the string, cross them, and pull tight to cut the dough into 12 equal pieces.
Place rolls cut side down into an oiled 9x13 inch pan or large baking sheet, being careful not to crowd the rolls so they have room to rise.
Cover with a dish towel, and let rise for 45 minutes to one hour until almost doubled in size.
Preheat oven to 350° F. Bake for 25-30 minutes (9x13 pan), until light golden brown on top. (Glass or ceramic pans take more time, dark pans & large baking sheets take less time, 22-25 minutes.) Cool 15-20 minutes before icing.
For the frosting:
In a medium bowl combine cream cheese, butter, and vanilla and mix until smooth. Mix in powdered sugar and milk until smooth. (Optional) add in peanut butter last. Drizzle over baked banana cinnamon rolls.
For the walnut topping:
In a frying pan over medium heat, warm butter and add walnuts, and toast for 2-3 minutes, stirring frequently. Stir in flour, remove from heat, then add in brown sugar last so it doesn't get too hot and caramelize and harden. Cool and sprinkle evenly over frosted banana cinnamon rolls, then serve!
Notes
Can be stored covered at room temperature for 2-3 days. Can be frozen once baked for up to 3 months. Overnight Instructions: Prepare recipe as directed to make the dough, let it rise, roll out and fill the dough, cut the rolls and set on a pan. Do not let rise a second time, cover and refrigerate overnight. They will rise slowly in the fridge. Preheat the oven in the morning and set the rolls out to rise for 30-40 minutes in a warm place (by the oven) and bake as directed.