In a large bowl or the bowl of a stand mixer, combine lukewarm milk, yeast, and 1 teaspoon sugar. Stir and let sit 10–15 minutes until foamy.

Add the remaining granulated sugar, salt, vanilla, and melted butter to the yeast mixture. Stir to combine.

Add half of the flour. Switch to the dough hook and mix on low until smooth.

Add the mashed banana and mix on low just until combined.

Gradually add remaining flour until dough forms. Knead on low (or by hand) about 5 minutes until smooth and slightly tacky but not sticky.

Place dough in a well-greased large bowl. Cover with plastic wrap and let rise in a warm, draft-free spot until doubled, about 1 hour.

Grease a 9×13-inch baking dish.

On a lightly floured surface, roll the dough into a rectangle (about 12×16 inches). Spread softened butter evenly across the surface.

Sprinkle brown sugar, cinnamon, and chopped walnuts evenly. Layer the sliced bananas in a single layer.

Roll the dough tightly from the long side into a log. Pinch the seam to seal and gently compress the log.

Slice the log into 12 equal pieces using dental floss or a sharp knife. Arrange rolls cut-side up in the prepared pan, leaving a little space between them.

Cover the pan with a clean towel and let rolls proof about 30 minutes until puffy.

Preheat the oven to 350°F (175°C).

Bake 30–35 minutes until the tops are golden brown and the center rolls are set. Remove from oven and let cool 10–15 minutes.

Beat room-temperature cream cheese and softened butter on medium speed until light and fluffy.

Add powdered sugar and vanilla. Beat until smooth and spreadable.

Spread frosting over warm rolls so it melts into crevices. Sprinkle with chopped walnuts and a pinch of ground cinnamon. Serve warm.