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Before you start, ensure the sourdough starter is very bubbly (if it’s not, feed it and wait until 1 tsp of the starter floats in warm water). Tip 200g of the starter into a large bowl with all but 2 tbsp of the egg, the milk, flour, sugar, cinnamon, mixed spice, orange zest and raisins. Mix with your hands until you have a shaggy dough (all the flour should be mixed in). Or, do this in a stand mixer fitted with a dough hook. Cover with a clean, damp tea towel and leave to prove for 30 mins. Cover the rest of the sourdough starter and keep chilled.
Work the butter and 1 tsp salt into the dough by squashing it in using your hands. Once fully combined, tip the dough onto a surface and knead gently for 5 mins until smooth and springy (again, this can be done in a stand mixer fitted with a dough hook). Form the dough into a ball, then return it to the bowl. Cover and leave somewhere slightly warm for 3-4 hrs to prove until the dough has almost doubled in size.
Tip the dough onto a floured work surface and knead briefly, then divide into 12 pieces, about 100g each. Roll the pieces into balls and arrange on a baking tray lined with baking parchment, leaving some room between each. Cover with a clean, damp tea towel, then leave to prove again for 2-3 hrs at room temperature until doubled in size, or chill overnight (this will result in better flavour and a neater shape).
If the buns have been chilled, remove from the fridge 1 hr before baking. Heat the oven to 200C/180C fan/gas 6. Brush the buns with the reserved egg. Tip the remaining starter into a piping bag fitted with a small nozzle, and pipe crosses onto the buns. Bake for 20-25 mins until light brown. Leave to cool slightly and serve warm, or cool completely. Best eaten on the day, but will keep for two days in an airtight container.