Dissolve the yeast in the water. In a food mixer with dough hook, place the flour, fermenté and butter. Pour in the yeasty water and mix to a solid dough. Mix on low for 4 minutes. Add the salt, then mix on medium speed for a further 2 minutes.
Remove the dough from the mixer and place in a bowl. Cover with a damp towel and leave until the dough has risen by two-thirds of its original s