For the dough: Place the flours into a large bowl. In a large jug, whisk together the yeast, sugar, salt, milk and egg. Then in a steady stream while mixing with your hand, pour the liquid mixture into the flours. Continue mixing/kneading until you have a smooth but sticky dough. Cover the bowl and rest in the fridge for 1 hour.
Meanwhile, using baking paper, fold to create an ‘envelope’ approx. 35x20cm. Then spread the softened butter on in a thin layer. Wrap the envelope and place into the fridge to chill.
Roll the chilled pastry, on a lightly floured work surface, into a large rectangle (approx. 55x30cm), with the short edge facing you. Then place the chilled butter over the bottom two thirds of the dough. Fold the top third of the dough (without any butter) down onto the middle third. Then fold the bottom third up on top of this. Roll the pastry out again into a large rectangle (again, short end facing you) and fold like before. This is your first turn. Wrap the dough, and chill in the fridge for 30 minutes. Repeat the turning process another two times, then leave in the fridge overnight to rest.
For the filling: The next morning, melt the butter. Then in a small bowl, mix the sugar, salt, spices and zest together. Using a pastry brush spread a thin layer of butter in 2 6-hole muffin tins, and a sprinkling of the sugar mix (approx. 1tsp across 4 muffin tin holes). Then set aside.
Roll the dough onto a lightly floured work surface, to approx. 60x40cm. Neaten up the edges with a knife to ensure your rectangle has straight edges. Then using a pastry brush, spread over the melted butter. Then scatter the sugar mix (leaving 50g to the side) over the butter, ensuring its an even layer and gently pressing into the dough so it sticks.
From the long edge, start rolling the dough up tightly until you have one long roll. Cut this into 12 even sections and place each into a muffin tin hole. Cover loosely, and leave to rise in a warm place for 1-2 hours, until doubled in size.
Preheat the oven to 200C. Then beat the egg in a small bowl, and using a pastry brush, glaze the tops of each bun. Place in the oven for 35 minutes, turning the tins round half way through, until they are fairly dark golden brown (you may need to cover with foil to stop them from burning). Tip the buns out onto a wire rack immediately (run a blunt knife around the edges if they get stuck) and leave to cool for 5 minutes.
Place the remaining sugar mix into a baking tray, then roll the hot buns in the sugar mix to coat.