Begin by greasing a bowl with some spare butter and setting this aside.
Sprinkle the yeast and 1 teaspoon of sugar into the warmed milk, stir and set aside for about 10 minutes. During this time the yeast should develop a foamy surface.
Meanwhile, in the bowl of a stand mixer fitted with a dough hook, add your flour, bread flour, cocoa powder, remaining sugar and salt. Give this a quick stir.
Once the yeast is ready, add this to the flour mixture, immediately followed by 2 whole eggs and one egg white. Begin kneading on a low setting, and continue for about 10 minutes until the dough has come together and is pulling itself from the sides of the bowl (see note).
After the 10 minutes, continue kneading on a low setting and begin adding the butter a few small pieces at a time.
Once all the butter has been added, continue kneading on a low setting for about 15 minutes. The dough should be starting to pull itself from the sides of the bowl and you should be able to touch it without it leaving significant residue on your finger.
Scrape the dough into your greased mixing bowl and cover with plastic wrap. Place this in the refrigerator to proof for about 8 hours (or overnight) until doubled in size.