Begin by warming the milk with half of the Nutella spread (1 tablespoon). This can be done in the microwave in bursts - stopping and stirring on 10 second increments until the Nutella is melted into the milk. Or, it can be done in a small saucepan over a medium heat, stirring occasionally until the Nutella dissolves.
Allow the milk to cool slightly to be about 104°F (40C) and then sprinkle the yeast over top. Give this a stir and then set aside for about 10 minutes until the yeast has developed a foam.
Meanwhile, place your flours, sugar and salt in the bowl of a stand mixer fitted with a dough hook. Give this a quick stir.
Add the yeast mixture to the flour, immediately followed by 2 whole eggs and one egg yolk (just to note: if you're adjusting the size of this recipe, these measurements will need adjusted too).
Turn the mixer to a low setting and knead for about 15 minutes, stopping occasionally at the beginning to scrape the sides of the bowl to ensure all the flour is incorporated. Knead until you have a dough that is beginning to pull itself from the sides of the bowl.
At this stage, begin to slowly add the butter a couple of small pieces at a time, with the mixer still on a low setting. Finally add the remaining Nutella. Knead on a low setting for a further 15 minutes until your dough is again pulling itself from the sides of the bowl.
Scrape the dough into a greased mixing bowl and cover with plastic wrap. Set in the refrigerator for a slow proof of about 8 hours (or overnight).