Begin by sprinkling the yeast and a teaspoon of sugar over the warmed milk. Stir this together and set aside for about 10 minutes for the yeast to become foamy.
Add the flours, remaining sugar and salt to the bowl of a stand mixer fitted with a dough hook. Give this a quick stir.
Once the yeast has activated (developed a foamy top), add this to the flour, immediately followed by 2 whole eggs and 1 egg white. Reserve the remaining egg yolk for an egg wash before baking. Place the remaining yolk in a small bowl and cover with plastic wrap to store in the refrigerator in the meantime.
Turn the mixer to a low setting and knead for about 15 minutes until you have a dough that is beginning to pull itself from the sides of the bowl. (see note)
After 15 minutes, keep the mixer on a low setting and slowly add your butter a few pieces at a time. Once these have all been added and are incorporated into the dough, continue kneading for about 15 minutes until the dough is again pulling itself away from the sides of the bowl. (see note)
Scrape the dough into a greased bowl and cover with plastic wrap. Place this in the refrigerator to proof slowly overnight (or for about 8 hours, until doubled in size).