Place the flour in the bowl of a stand mixer; add the yeast to one side and the salt and sugar to the other.
Add in the mashed banana, melted butter and milk and mix on a low speed until it comes together into a dough. Turn up the speed to high and mix until the dough starts to pull away from the sides of the bowl cleanly and is very elastic; it should pass the windowpane test.
Transfer the dough to a large, lightly greased bowl, cover with clingfilm and place in the fridge to rise overnight. (Or in a warm place until doubled in size - 1-2 hours).
Line an approx 9.5x5in loaf tin with baking parchment, leaving some overhanging to make it easier to lift the baked loaf out of the tin. Punch down the risen dough and turn it out onto a lightly floured surface; sprinkle with flour and roll out to a large rectangle, about 30x45cm.
Spread the nutella evenly all over the dough, leaving a small border around the edge. roll the dough up tightly from one of the long edges into a log.
Use a sharp knife to slice the log in half lengthwise. Twist the two halves together, tuck the ends under and carefully lift it into the loaf tin.
Loosely cover with oiled clingfilm and set aside to rise until puffy, about 45mins to 1 hour. Meanwhile, preheat the oven to 190C/375F/gas mark 5.
Bake the risen dough for 60-70 minutes, until the internal temperature reaches at least 94C/201F on a probe thermometer. You will probably have to cover the top with tin foil partway through baking to prevent it from catching.
Leave the bread to cool in the tin for 10 minutes then carefully lift it out onto a wire rack and leave to cool before slicing.