Begin by sprinkling your yeast and 1 teaspoon of the sugar over your warmed milk. Give this a stir and set aside until it becomes foamy (about 10 minutes).
Meanwhile, add the flour, bread flour, remaining sugar and salt to the bowl of a stand mixer fitted with a dough hook. Stir this together.
Once the yeast is activated, add this to the flour mixture immediately followed by 2 eggs and one egg white. Reserve the remaining egg yolk to make an egg wash before baking.
Turn the mixer on to a low setting and knead for about 15 minutes, stopping early on to knock any flour off the sides of the bowl and into the wet dough as it comes together.
After 15 minutes, you should have a sticky wet dough that is showing signs of pulling itself away from the sides of the bowl.
Keeping the mixer on a low setting, slowly add the butter a few pieces at a time.
Once all the butter is added, continue kneading for 15 minutes until the dough is again slapping the sides of the bowl and starting to pull itself away. It is a wet dough and will likely continue to stick, especially to the base of the bowl.
Scrape the dough into a greased mixing bowl and cover with plastic wrap. Place in the refrigerator to proof slowly overnight (or for about 8 hours).