Mix the dry ingredients. In a large bowl, whisk the flour, yeast, sugar, salt, and cinnamon.
Set aside.
Combine wet ingredients. In a separate bowl, mix the warm milk, mashed banana, egg, melted butter, and vanilla. The milk should feel warm but not hot.
Form the dough. Pour the wet ingredients into the dry. Stir with a spoon or spatula until a shaggy dough forms.
If it feels too sticky, add 1–2 tablespoons of flour. If too dry, add a tablespoon of milk.
Knead until smooth. Turn the dough onto a lightly floured surface and knead for 5–7 minutes, until smooth and elastic. You can also use a stand mixer with a dough hook for about 4–5 minutes on medium-low.
First rise. Place the dough in a lightly oiled bowl, cover, and let it rise in a warm spot until doubled, about 60–90 minutes.
It should look puffy and spring back slowly when pressed.
Prepare the filling. In a small bowl, stir together the softened butter, brown sugar, cinnamon, and a pinch of salt until it forms a spreadable paste.
Roll out the dough. Punch down the dough and roll it into a rectangle about 12x16 inches. Spread the filling evenly over the surface. Sprinkle nuts on top if using.
Roll and slice. Starting from a long side, roll the dough into a tight log.
Trim the ends. Slice into 12 even rolls using dental floss or a sharp knife to keep the swirls neat.
Second rise. Arrange rolls in a greased 9x13-inch pan (or two 9-inch round pans), leaving a little space between them. Cover and let rise until puffy, 30–45 minutes.
They should expand and touch.
Bake. Preheat the oven to 350°F (175°C). Bake for 20–25 minutes, until lightly golden on top and the centers register about 190°F (88°C). Do not overbake or they’ll dry out.
Make the frosting. Beat the cream cheese and butter until smooth.
Add powdered sugar, vanilla, a pinch of salt, and 1 tablespoon of milk. Beat until fluffy, adding more milk for a softer glaze.
Frost and serve. Let the rolls cool for 10 minutes, then spread with frosting while still warm so it melts slightly into the swirls. Serve fresh.