In a small saucepan, combine 20g of flour and 100ml of milk. Cook over medium-low heat, stirring constantly until the mixture thickens into a smooth paste. Remove from heat, cover, and set aside to cool.

In a large mixing bowl, combine 480g flour, 3g salt, 45g cocoa powder, 5g instant yeast (or activated 10g active dry yeast), and 100g granulated sugar. Mix briefly to combine.

If using active dry yeast, dissolve it in 200ml of lukewarm milk and let it sit for about 10 minutes until foamy, then add to the dry ingredients. If using instant yeast, add the milk directly to the dry ingredients along with one large egg and the cooled tangzhong paste.

Mix the ingredients using a dough hook attachment on a stand mixer or by hand until there are no dry patches of flour visible. Cover the dough and let it rest for 10 minutes.

After resting, add 42g of softened butter to the dough and knead for 10-15 minutes until it becomes soft and smooth. The dough should be tacky but not sticky. Avoid adding extra flour during this process.

Shape the dough into a ball and place it in a greased bowl. Cover and let it rise in a warm place for 1 to 1.5 hours, or until it has doubled in size.

While the dough is rising, prepare the filling. Combine 70g softened butter, 70g dark brown sugar, and 7g ground cinnamon in a bowl. Mix until smooth and spreadable. If necessary, microwave for a few seconds to soften further.

Once the dough has risen, turn it out onto a lightly floured work surface. Using a rolling pin, roll the dough into a rectangle approximately 14x16 inches (35x40cm).

Spread the cinnamon-sugar filling evenly over the rolled-out dough, leaving a small border around the edges. Starting from one short end, tightly roll the dough into a spiral log.

Using dental floss or a very sharp knife, trim about 1/4 inch off each end of the dough log. Slice the remaining dough into 9 equal pieces (about 1.5 inches thick) and place them in a greased 14x14 inch square pan. If using a 9x13 inch pan, you can stretch the dough slightly and cut thinner pieces to make 12 rolls. Cover the pan and let the rolls proof for 30-45 minutes, or until they are puffy and a slight indentation remains when gently pressed with a finger.

While the rolls are proofing, preheat your oven to 355°F (180°C).

Bake the rolls for 25-27 minutes, or until they are golden brown and cooked through.

As the rolls are baking, prepare the Nutella icing. In a small bowl, combine 2 tbsp Nutella, 30g powdered sugar, 1/2 tsp vanilla extract, and 30ml milk. Stir until smooth, adding more milk if necessary to reach the desired consistency.

Once the rolls are out of the oven, let them cool for a few minutes. Drizzle the Nutella icing over the warm rolls. Allow them to cool slightly before serving.