1. Prepare the sponge with the yeast, 100g of the flour and warm water. Mix it well and leave in a warm place for about 2 hours.
2. When it’s risen and bubbling, add the remaining flour, the milk, salt, sugar and beat in a mixer with a dough hook attachment, or just whisk together and then knead by hand until smooth and stretchy - the dough will be stiff and firm. Cover the bowl with cling film and keep in the fridge overnight.
3. The next day prepare the butter – cool but not straight from the fridge, place it between two sheets of cling film dusted with flour. Roll or bash it down to a thickness of about 1cm.
4. Roll out the dough, also to about 1 cm thick. Place the butter in the centre, fold all the sides over it so it’s completely enclosed.
5. Dust the dough with flour and roll out to a large rectangle (60 x 40 cm would be good but the dough springs back so much that as large as it gets will be fine). Now fold the two ends by a sixth each, then once again to make them meet in the middle. Fold the two together like a book. Turn the dough with the fold to one side and roll out again, as large a rectangle as you can. Fold each end by a quarter so that they meet in the middle, and then together like closing a book again.
6. Chill in the fridge for up to 1 hour.
7. Prepare a triangular template about 15cm at the base and 15cm tall, or just trust your ability to cut even triangles with a knife or pastry cutter.
8. Roll out the chilled dough to about 4mm thickness, trim the edges and divide into strips 15cm high. Cut triangles then roll each one up from the base, finishing with the point in the middle and underneath. Place them on baking trays lined with parchment, cover with cling film and leave in a warm place to prove for about 1 – 1 ½ hour.
9. Preheat the oven to 200C/400F/gas 6. Glaze the croissants with the egg yolk beaten with milk, then bake for about 10 minutes. Reduce the heat to 180C/350F/gas 4 and bake for further 20-25 minutes until risen and golden brown.