Warm the buttermilk until comfortably warm but not hot. Stir in the yeast with a small amount of the granulated sugar and let it stand until the yeast becomes active.

Combine the mashed banana, remaining sugar, egg, melted butter, and vanilla in a large mixing bowl. Add the yeast mixture and stir until combined.

Gradually add the flour and salt. Mix until a soft dough forms. Knead until the dough becomes smooth and elastic. The dough should remain slightly tacky rather than dry.

Place the dough in a lightly greased bowl. Cover and leave it in a warm place until noticeably increased in volume, about 60 to 90 minutes depending on room temperature.

Prepare the filling by combining the softened butter, brown sugar, cinnamon, mashed banana, and optional nutmeg.

Roll the risen dough into a large rectangle on a lightly floured surface. Spread the cinnamon filling evenly over the dough. Sprinkle with chopped walnuts or pecans if using.

Starting from a long edge, roll the dough into a tight log. Cut into 12 evenly sized rolls.

Arrange the rolls in a greased baking dish. Cover and allow them to rise again until soft and puffy, approximately 30 to 45 minutes.

Preheat the oven to 350°F or 175°C. Bake for approximately 22 to 27 minutes, or until the rolls are golden and cooked through.

While the rolls bake, beat the cream cheese and butter until smooth. Add the powdered sugar and vanilla. Mix in enough buttermilk to achieve a smooth, spreadable frosting.

Allow the baked rolls to cool slightly, then spread the cream cheese frosting over the warm rolls and serve.