Mix together the mashed banana, milk and sugar in the bowl of a stand mixer fitted with a dough hook.
Add the flour, yeast and salt and mix on a low speed to form a rough dough. Continue mixing on a medium speed until the dough is smooth and stretchy.
Add the butter and continue to knead on a medium speed until it is fully incorporated. It will look greasy and messy at first but it will come together eventally! Scrape down the sides of the bowl a few times as necessary.
The dough should be sticky but not wet, should pull away from the sides of the bowl cleanly, and you should be able to stretch it to a thin windowpane without it tearing. If it is too wet you can add a bit more flour, a spoonful at a time. Be careful though, the dough should be a bit sticky.
Place the dough in a lightly oiled bowl, cover and set aside to rise in a warm spot until doubled in size, about 1-2 hours.
When the dough has risen, thoroughly grease a 10-12 cup bundt tin. Melt the butter and set aside to cool. Mix together the sugar and spices in a wide, shallow bowl.
Spoon 2 Tablespoons of melted butter, 3 Tablespoons of spiced sugar, and 4 Tablespoons of chopped nuts into the base of the tin.
Punch down the risen dough and give it a brief knead to knock out the air. Tear small pieces of dough off and roll them into balls. You want to end up with around 55-65 balls of dough so keep them small.
Working with a few at a time, dunk the balls of dough in the melted butter, then roll them in the spiced sugar, making sure that they are fully coated in both.
Place the coated balls in the tin and repeat until you have a full layer of dough in the tin. Scatter over most of the rest of the chopped nuts, then continue to layer the coated dough balls in the tin.
Once you have used all of the dough, scatter over the last of the nuts and drizzle over any remaining butter and spiced sugar.
Cover the tin and leave to rise in a warm place for 45-60 minutes. It is ready to bake when the dough is puffy and springs back slowly, leaving a small indentation, if you gently poke it with a finger. If it springs back quickly and fills in completely then it needs to rise for longer.
When the dough is nearly ready, preheat the oven to 180°C/160°C fan/350°F/gas mark 4.
Bake for 35-45 minutes until nicely browned and crisp on top. A probe thermometer inserted into the centre should reach at least 90°C/194°F.
Allow to cool in the tin for about 20-30 minutes before turning it out onto a serving plate. You may need to give the tin a shake to release it. Best served warm and freshly baked.