Activate the yeast. Combine the lukewarm milk, yeast, and granulated sugar in a bowl. Stir and let stand until the mixture begins to foam.
Make the dough. Sift the flour and cocoa powder into a large bowl and stir in the salt. Add the yeast mixture, eggs, and softened butter. Mix until combined, then knead until the dough is smooth and elastic.
First rise. Cover the bowl and let the dough rise in a warm spot for about 60 minutes, or until doubled in size.
Roll out and fill. Transfer the dough to a lightly floured work surface and roll into a thin rectangle. Spread evenly with the softened filling butter. Mix the cinnamon and brown sugar, then sprinkle evenly over the butter.
Shape the chocolate cinnamon rolls. Roll the dough into a log, starting from a long side. Use clean thread or unflavored dental floss to cut into 12 rolls, approximately 1¼–2 inches (3–5 cm) thick.
Second rise. Arrange the rolls in a lightly buttered baking dish. Cover and let rise for about 30 minutes, until noticeably puffy. Toward the end of the rise, preheat the oven to 350°F (180°C).
Bake. Bake for 20 minutes, until the rolls are baked through in the center. Remove from the oven and let cool slightly.
Make the frosting. Beat the softened butter and powdered sugar until smooth. Mix in the milk and salt, then add the Nutella and mix until evenly combined.
Frost and serve. Spread the frosting generously over the slightly warm chocolate cinnamon rolls. Serve warm.