Mix together the dough ingredients and leave to rest for 1h30 - 2h. Take the butter out of the fridge and leave at room temperature while the dough rests.
When the dough has rested, roll it out into a rectangle approx 30x40 cm.
Spread the softened butter (with your hands is easiest) over the surface of the dough. Mix the remaining filling ingredients (sugars and cinnamon) together in a bowl, and spread evenly over the buttered dough.
Roll the dough up and cut into 12 slices. Place them on a tray (with some spacing between them as they will rise and expand while cooking) and leave to rest in the fridge overnight (up to 12h).
Take the tray out of the fridge and leave for approx 1h to regain room temperature (to ensure an even bake).
Brush the buns with milk, then bake for 15-20 mins at 160°C in a fan oven (or 180°C in a conventional oven), or until golden brown.
Brush with honey, dust with icing sugar, or drizzle with icing.