Lightly grease a bowl with butter or flavorless oil and set aside.
Next, sprinkle the yeast and a teaspoon of sugar over the warm milk. Give this a stir and leave to sit and activate for about 10 minutes until it is very foamy.
Meanwhile, place the flours, remaining sugar and salt in the bowl of a stand mixer fitted with a dough hook and give the mixture a quick stir.
When your yeast is ready, add this to the flour mixture, followed by two eggs and one egg white. Reserve the remaining egg yolk for an egg wash when baking.
Knead the dough on a low setting for about 15 minutes until it begins pulling itself away from the sides of the bowl. (see note)
While continuing to knead, slowly add the butter a few pieces at a a time, and then add the peanut butter in a couple of spoonfuls.
Now that all of your ingredients are added, continue kneading for another 15 minutes until the dough is again coming together. It should be fairly sticky, but should be slapping the sides of the bowl, even if some residue is being left on the sides.
Scrape this dough into your greased bowl, cover with plastic wrap and place in the refrigerator for a long slow proof. This should be about 8 hours (or overnight), until doubled in size.