In a bowl, place the beer and the instant yeast and do not disturb it for 15 minutes. It will start to froth.
Add the flour, sugar and egg yolk, and knead until elastic.
Add the salt and butter, and knead until the dough passes the window pane test.
Shape into french loaves.
Top with egg wash and coarse salt (optional).
Place them in a tray and into the oven with a pot of boiling water or in a steam oven set at 30C.
When the shaped bread have doubled in size, bake at 190C on the middle shelf for 13-15 minutes.