Begin by greasing a bowl with some extra butter and set aside.
In a small bowl, sprinkle the yeast and a teaspoon of sugar over top the warmed milk. Give this a light stir and set aside to allow the yeast to activate. This should develop a good foam on top.
Meanwhile, add the flours, the remaining sugar and the salt to the bowl of a stand mixer fitted with a dough hook. Stir together lightly.
Once the yeast is active, add this to the flour mixture, immediately followed by two whole eggs and one egg white. The leftover yolk can be reserved for an egg wash when ready to bake.
Knead with the mixer on a low setting for about 15 minutes, until you have a sticky dough, that is showing signs of holding itself together and slapping the sides of the bowl.
At this stage, with the mixer still continuing to knead, add the butter a couple of pieces at a time until it is all incorporated.
Then pause the kneading to add a few drops of blue food colour - the amount will depend on the type of food dye you are using. A couple of drops of gel food colour is usually perfect here.
Return to kneading at a slow speed for a further 15 minutes until the dough is again showing signs of pullings itself from the sides of the bowl. It should also be quite blue!
Scrape the dough into the greased bowl, cover with plastic wrap and place in the refrigerator to proof overnight (or for about 8 hours) until doubled in size.