Heat the milk in a saucepan until finger-warm (no more than 37°C/98°F), then add the ground saffron.
In a stand mixer, add the fresh yeast and the milk-saffron mixture (again, no warmer than 37°C/98°F or the yeast will die). Mix for one minute, then add the sugar and stir until dissolved. Stir in the quark or yoghurt until incorporated, then mix in about half the flour, as well as the salt. As you keep mixing, gradually add more flour, taking care not to add too
much (saffron is very drying, so if you have a dry dough, the end result will also be dry). Add the butter and half of the egg and keep mixing, adding more flour as needed. This will take around five minutes.
When the dough is springy and well-kneaded, leave to rest in a covered bowl in a warm place for about 40 minutes or until doubled in size.
Turn out the dough onto a floured surface and knead. Cut into 16 even pieces. Roll each piece into a 20cm-long roll. Take each end and twist them back in on themselves in opposite directions, so you end up with an S-shape. Line a baking tray with paper, then place each bun on it, ensuring there is a good distance between each one (or you can shape into a wreath by placing the buns in a circle, leaving a 1cm gap between them, as they spread during baking). Gently press a raisin into the centre of each swirl. Leave to rise for 20 minutes.
Preheat the oven to 170°C/340°F/Gas 3½.
Gently brush each bun with the remaining egg and bake in the preheated oven for eight to 10 minutes, or until golden and done. Leave under a damp tea towel for at least 10 minutes as soon as they come out of the oven to ensure no crust forms. If you wish, scatter over some pearl sugar.
Saffron dough dries out quickly, so either eat the buns on the day of baking or freeze as soon as they’re cool. You can also enjoy them slightly toasted when they’re a few days old.