4: Go back to the slowest speed, then gradually trickle in the butter.
5: Knead until the butter is completely mixed in.
6: Finish by grating the zest of 1 orange and 1 lemon into the mixture and knead for a further 2 minutes to mix in.
7: Gather the dough into a ball and transfer to a large, clean bowl. Cover with a plastic sheet and leave to rest at room temperature for 2 hours.
8: Divide the dough into lumps of about 800 g and shape into balls. Sit these on a baking sheet, cover with a plastic sheet and leave to rest in the fridge overnight.
9: Next day, reshape each ball and put into a tall non-stick tin or mould. Cover with a plastic sheet.