Puree bananas in a food processor along with 1 tsp lemon juice until mixture is smooth and no chunks remain, set aside (you should have just over 1 1/3 cups pureed bananas).
Heat milk and 3 Tbsp butter in a small saucepan over medium low heat. Cook stirring occasionally until butter has melted. Remove from heat and pour mixture into the bowl of an electric stand mixer. Whisk in vegetable oil and allow mixture to cool to 110 degrees. Whisk in yeast, stirring until dissolved and allow mixture to rest 5 minutes.
With the paddle attachment, mix in granulated sugar, salt, egg yolk, 2 cups bread flour and pureed bananas. Switch mixer to a hook attachment, add 1 1/2 cups bread flour and sprinkle in cornstarch, then knead mixture on medium low speed until a smooth and elastic dough forms (dough will be sticky). Cover bowl with plastic wrap and allow to rise to until doubled in size, about 1 1/2 hours.
In a small bowl whisk together light-brown sugar, cinnamon and nutmeg, set aside.
In a separate small bowl, whisk together remaining 1/4 cup bread flour and baking powder. Return bowl with dough to electric stand mixer, pour in flour/baking powder mixture and knead dough with hook attachment until mixture is well blended and smooth.
Roll dough out on a floured surface (dough should be slightly sticky so I floured the surface fairly generously) to a 16 x 20 inch rectangle. Using the back of a spoon, spread 6 Tbsp melted butter evenly over the top of the dough. Sprinkle top evenly with brown sugar mixture and spread into an even layer, coming within a 1/2 inch of all edges. Starting on the 16 inch side, snugly roll dough to opposite end.
Cut rolled dough into 12 equal pieces (I cut it in half first, then halves into halves, then those halves into thirds). Arrange rolls in a buttered 13 x 9 inch baking dish (reshaping flat sides as needed).
Cover baking dish with plastic wrap and allow to rise until doubled in size, about 45 minutes. Preheat oven to 350 degrees (I do this during the last 10 minutes of rising) and bake rolls in preheated oven for 22 - 26 minutes until golden. Allow to cool slightly then cut into rolls and frost with Cream Cheese Frosting and sprinkle with optional chopped nuts.
Cream Cheese Frosting
In a large mixing bowl, using an electric and mixer, whip together cream cheese and butter until light and fluffy, about 2 minutes. Add powdered sugar and mix until smooth. Pour in milk and mix until well blended.