Begin by sprinkling the yeast and 1 teaspoon of your sugar over your warmed milk. Stir and set aside for about 10 minutes until active and very foamy. (see note)
Meanwhile, in the bowl of a stand mixer fitted with a dough hook, stir together the flour, bread flour, salt and remaining sugar.
When the milk and yeast is ready, add this to the flour mixture, immediately followed by two whole eggs and 1 egg white. Reserve the final egg yolk for an egg wash when you are prepared to bake.
Knead the mixture for 15 minutes, until a dough has formed and this is beginning to pull itself from the sides of the bowl. (see note)
Slowly add the softened butter, a bit at a time, continuing to knead as you go.
Once all the butter is incorporated, knead for 15 more minutes until the dough is again pulling itself from the sides of the bowl.
Scrape the dough into a greased bowl, cover with plastic wrap and set in the refrigerator to proof overnight (or for 8 hours).