In a large mixing bowl, combine the bread flour, sugar, egg, salt, and milk. Mix with a spatula or wooden spoon until a dough forms with no dry flour.
Tightly cover the bowl with plastic wrap and place it in the refrigerator to rest for 4 hours or overnight (up to 12 hours). This slow, cold rest hydrates the flour and begins gluten development without kneading.
Before you're ready to work with the dough, prepare the yeast. In a small bowl, add 1 1/2 tsp of water to the yeast, and mix until it forms a paste.
Remove the dough from the fridge. It will be firmer and more developed. Stretch it gently into a flat dough.
Smear the yeast paste across the surface of the dough. Begin kneading by hand for about 3 minutes until the yeast is fully incorporated.
Once the yeast is fully mixed in, add the softened butter to the dough. Continue to knead for another 5 minutes. At first, the dough will become slippery and may fall apart, but keep kneading until the butter is fully absorbed and the dough becomes smooth and soft. The dough should not feel sticky.
Form the dough into a ball, and cover with a cloth or plastic wrap. Let it rise in room temperature for about 1 hour, or until doubled in size.
While the dough is proofing, prepare the Nutella using a tablespoon. Scoop six equal portions of Nutella onto a piece of parchment paper, and freeze them in the freezer.
Deflate the dough to release the gas, then pour the chocolate chips over it. Fold the two sides to the middle, roll it up, and gently knead the chocolate chips into the dough.
Divide the dough into 6 pieces, shape them into balls, and let them relax for 10 minutes.
Take one ball and flatten it with your hands or use a rolling pin to roll it out. Place 1 frozen Nutella piece in the center, and pinch the edges to seal it. Repeat with the remaining balls.
Place the doughs in the baking sheet, cover, and let them proof for 40 minutes.
While the dough proofs, preheat your oven to 190°C (374°F).
Gently brush the top with milk
Bake for 12-15 minutes, or until the top is a deep golden brown and the internal temperature registers at least 190°F (88°C).
Dust the cocoa powder on top of the buns.
Remove the buns from the pan and transfer them to a wire rack. Allow it to cool completely before serving.
