Put water, 1 tsp sugar and yeast in a bowl and mix until dissolved. Add 50g flour and mix briefly. Form a ball, cover the bowl with a damp cloth and let the pre-dough rise in a warm place for about 30 minutes.
Add the remaining 550g flour, milk, 1 tsp sugar, butter and salt to the pre-dough and knead everything with a kneading machine or food processor or by hand until a smooth dough is formed. Put the dough in a large bowl, cover with a damp cloth and let it rise until doubled (about 2 hours).
For braiding, braid.
If you want to bake it on the same day: Place it on a baking tray covered with baking paper, brush with a little water and let it rise until doubled in size (about 1 hour).
If you want to bake it the day after: Place the braid on a baking sheet lined with parchment paper, brush with a little water, cover with foil, and let it rest in the refrigerator overnight (about 12-14 hours).
Before baking, brush with cream or milk
Set oven temperature to 200 °C convection with 100% steam. Bake in the center of the oven on rising heat for 35-40 minutes until golden brown.
Leave to cool for 2 hours before serving.
