Mix the flour, yeast, milk and a pinch of salt in a tabletop mixer with a dough hook. Mix in two eggs and the vanilla, then knead until you have a smooth, springy dough. Cover and leave to rise for at least 2 hrs, or overnight in the fridge, until doubled in size.
Start the mixer again and add the sugar, butter and yolk from the remaining egg. Keep mixing and kneading until the dough is smooth again. Tip in the fruit and knead again quickly. Cover and leave to rise somewhere cool or in the fridge (so the butter doesn’t melt out of the dough) for at least two hours, or overnight in the fridge, until doubled in size.
Line a large six-hole muffin tin with paper cases. Divide the dough into six portions and roll into neat balls. Put a ball in each paper case, cover the tin with a tea towel and leave the dough to rise for 20 mins. Alternatively, chill in the fridge overnight, then leave the dough balls to warm up for 30 mins before baking.
Heat the oven to 180C/160C fan/gas 4. Brush the tops of the muffins with the egg white and dust with caster sugar. Bake for 15-20 mins, or until the muffins have risen and are golden brown in colour. Leave to cool on a wire rack. Will keep for three days in an airtight container.