Place the blade guard on. Place 250g of milk (keep 10g of milk aside), yeast and sugar in the bowl. Heat to 37°C on speed 2 for 6 minutes.Add the diced butter and 1 egg. Mix for 3 minutes at 37°C on speed 2.Add flour and salt. Mix for 4 minutes and 30 seconds on speed 3.Remove the dough from the bowl and leave to rest for 1 1/2 hours in a bowl covered with a tea towel.On a floured work surface, work the dough to de-gas it. Divide the dough into 2 balls. Divide each ball into 3 and form 3 balls of 35 cm. On a baking tray covered with baking paper, braid the strands. Make 2 plaited buns. Leave to rest for 60 minutes covered with a clean tea towel.In a small bowl, mix one egg yolk with 100ml milk. Brush both buns with the egg and milk mixture. Sprinkle pearl sugar over the buns (optional).Bake for 30 minutes at 180°C without preheating the oven. Take the buns out when they are golden brown.