Sprinkle your yeast and a teaspoon of your brown sugar into your warmed milk and set aside for 10 minutes to allow it to activate.
Meanwhile, place the flours, remaining brown sugar and salt in the bowl of a stand mixer fitted with a dough hook. Give it a quick stir.
Once the yeast has become foamy, add this to the flour, immediately followed by two whole eggs and 1 egg white. Reserve the remaining egg yolk to make an egg wash before baking.
Turn the mixer onto a low setting and knead for 15 minutes, stopping occasionally at the beginning to ensure all of the flour is being incorporated.
After 15 minutes, your dough should have come together and be starting to pull itself from the sides of the mixing bowl. Do note that it will still be a wet and sticky dough.
Slowly add the butter a few pieces at a time, while the mixer continues to knead on a low speed.
Once all of the butter is added, knead for a further 15 minutes until the dough has come together again and is showing signs of pulling itself from the sides of the bowl. This will stay a very wet dough, so don't worry if it is still sticking to the sides and bottom and is very tacky to the touch.
Scrape the dough into a greased mixing bowl, cover with plastic wrap and set in the refrigerator to proof for about 8 hours, or overnight - it should double in size.