Start by greasing a bowl with butter or flavorless oil and setting aside.
Then sprinkle the yeast and a teaspoon of sugar into your warmed milk. Give this a quick stir and set aside for about 10 minutes. During this time, the yeast should activate and become pretty foamy.
Meanwhile, place the flours, remaining sugar, salt and cocoa powder in the bowl of a stand mixer fitted with a dough hook. Stir this together lightly.
When the yeast is ready, pour this into the flour mixture and add your eggs (two whole eggs and one egg white). Begin kneading on a low setting, scraping the sides of the bowl down after a minute or two to ensure all of the dry ingredients are being incorporated.
Knead for about 12-15 minutes until you have a sticky dough that seems to be holding itself together.
After this first kneading process, keep the mixer on a low setting and add the butter (carefully) a small bit at a time. Once all the butter is added, knead for another 10-12 minutes until your dough is once again showing signs of pulling itself away from the sides of the bowl.
Add the chocolate chips and knead for a couple more minutes to get them as combined as possible.
Scrape the dough into your greased bowl, cover with plastic wrap and place in the refrigerator to proof slowly overnight (or for about 8 hours until doubled in size).