Begin by sprinkling the yeast and 1 teaspoon of sugar into the warmed water. Give this a stir and set aside for about 10 minutes to allow the yeast to activate and become foamy.
In the bowl of a stand mixer fitted with a dough hook, stir the flour, bread flour, remaining sugar and salt together.
When the yeast is ready, pour this into the flour, immediately followed by 2 whole eggs and 1 egg white. Reserve the remaining egg yolk to make an egg wash when ready to bake.
Knead the dough on a low setting for 15 minutes until your dough has come together and is starting to pull itself away from the sides of the bowl. It will stay a very wet dough, so don't expect it to pull itself clean.
Add the pieces of butter a few pieces at a time, continuing to knead on a low setting. Once all of the butter is incorporated, knead for a further 15 minutes until the dough has again come together and is starting to pull itself from the sides of the bowl. Again this will stay a wet and sticky dough, so won't pull itself completely clean (especially from the bottom of the bowl).
Scrape the dough into a greased bowl, cover with plastic wrap and place in the refrigerator to proof slowly overnight (or for 8 hours).