Pour the lukewarm milk into a large bowl and stir through the yeast and sugar. Leave to stand for a few minutes.
Add the melted butter, egg and mashed banana and whisk together until well combined.
Add the flour and salt then, using a wooden spoon, mix together to form a dough.
Lightly dust a work surface with flour and turn the dough out on top. Knead for about 8-10 minutes - or until smooth, elastic and can easily be shaped into a ball (it should feel slightly tacky but not stick to your hands or the work surface). If it is still too sticky, add 1 tbsp of flour and continue kneading for a few minutes until it becomes a smooth, elastic ball (add a little more flour if still needed).
Pop the dough into a lightly greased bowl, cover with a clean cloth or cling film and leave in a warm place to rise for about 1 hour - 1 ½ hours - or until doubled in size.
Meanwhile, make the filling: in a medium bowl mix together the butter, sugar and cinnamon until well combined. Cover with cling film and set aside.
Grease a casserole dish - you need to be fairly deep. Set aside.
Once the dough has risen, punch it down and transfer to a lightly floured surface. Shape into a rough rectangle.
Lightly dust a rolling pin with flour, then roll out the dough to a large 40x20cm rectangle about 5mm thick.
Spread the cinnamon butter evenly over the dough, then roll up tightly from the long side as you would a Swiss roll. Place seam-side down.
Using a sharp knife, cut into 12 equal-sized pieces. Carefully place into the prepared casserole evenly spaced apart - you should have 3 rows with 4 rolls per row (alternatively, pop onto the prepared baking sheet). Cover with a clean cloth or cling film and leave to rise in a warm place for another 20-25 minutes or until they have almost doubled in size.
Meanwhile, preheat the oven to 175C Fan.
Once the rolls have almost doubled in size, bake for about 25-30 minutes - or until lightly golden and the base sounds hollow when tapped. Leave to cool for 10 minutes.
For the frosting: put the butter, cream cheese, maple syrup and banana into a blender and blitz until smooth. Taste and add lemon juice, if desired. Blitz again to combine.
Drizzle the frosting over the slightly warm rolls and serve.