Soak the raisins in boiling water and set aside to cool down. This can be done up to three days ahead, in which case, keep the soaked raisins in the fridge.
Slice the butter into thin 2-3mm thick slices. Set aside until needed.
In the bowl of a stand-mixer fitted with the dough-hook, place the flour, sugar, yeast, saffron and salt. Add the milk and mix on medium speed for around 10 minutes or until the dough detaches from the sides of the bowl and feels smooth, elastic and barely tacky. If you take a small piece of dough, you should be able to stretch it into a very thin membrane.
Add the butter, one small piece at a time continuously until all the butter is in – and knead it in for a further 10 minutes.
Place the dough in a large bowl, and clingfilm to the touch. Refrigerate for at least 3 hours or up to 12.
Line three baking trays with paper and set aside.
Scrape the dough onto a lightly floured surface and divide in 50-55g pieces and cover loosely with clingfilm.
Take one piece and roll into a thin snake, approximately 30cm long, then form an S shape, curling both ends into a spiral. Place onto the prepared baking trays, making sure to give the buns plenty of space. And repeat with the remaining dough.
Cover with clingfilm and leave to proof until doubled in size, around 2-3 hours.
Preheat the oven to 200°C/fan 180°C.
Brush the top of the buns with the egg wash and press two raisins into each bun.
Bake for 10-12 minutes until golden-brown. Allow to cool down slightly.