In a large bowl mix the warm milk, sugar, salt, butter, egg yolk and 200 g flour.
Mix with the hook for 1 minute.
Add the yeast and mix for 2 minutes again.
Add the remaining 300 g flour and work with the hook.
When the dough has pulled together and becomes smooth and elastic (after about 5 minutes) you can form the braid.
Divide the dough into 2 equal pieces and roll each piece into a 60 cm long cylinder (or 3 pieces of 30 cm). Braid the pieces together and place on a lightly greased baking sheet. This is the method that I use: http://www.youtube.com/watch?v=lcPnB0Sgc-Q.
In a small bowl, mix egg white and water and brush the bread with this mixture.
Let the dough rest after forming for 90 minutes until it doubles in size.
Bake in the preheated oven at 180°C for 20 to 25 minutes (heat and timing are basing it on convection oven), until golden.