01 - Combine flour, cocoa powder, instant yeast, granulated sugar, and salt in a large mixing bowl. Whisk thoroughly to distribute ingredients evenly.
02 - Pour warm milk, melted butter, and cracked egg into the dry ingredients. Stir with a wooden spoon until a sticky, shaggy dough forms.
03 - Knead the dough by hand on a lightly floured surface or with a stand mixer fitted with a dough hook for 8-10 minutes until smooth, elastic, and no longer sticky.
04 - Transfer the dough to a greased bowl, turning once to coat all surfaces. Cover with plastic wrap or a clean kitchen towel. Let rise in a warm, draft-free area until doubled in bulk, approximately 1 hour.
05 - Punch down the risen dough to release air. Roll out on a floured surface into a 16x12-inch rectangle, maintaining even thickness throughout.
06 - Spread softened butter evenly over the entire surface of the rolled dough, leaving a 1/2-inch border around edges.
07 - Combine brown sugar, cocoa powder, and ground cinnamon in a small bowl. Mix until no lumps remain and the mixture is uniformly colored.
08 - Sprinkle the sugar-cocoa mixture evenly over the buttered dough. Scatter dark chocolate chips across the surface, pressing gently to adhere.
09 - Starting from one long edge, roll the dough tightly into a cylinder. Pinch the seam firmly to seal. Using a sharp knife or dental floss, slice the log into 12 equal portions.
10 - Place the rolls cut-side up in a greased 9x13-inch baking dish, leaving space for expansion. Cover loosely and let rise in a warm area until puffy and doubled, 30-45 minutes.
11 - Preheat the oven to 350°F while the rolls complete their final rise.
12 - Bake the risen rolls for 22-25 minutes until set and lightly golden brown on top. The filling should be bubbly and the dough fully cooked through.
13 - While rolls cool slightly in the pan, whisk powdered sugar, cocoa powder, 2 tablespoons milk, and vanilla extract in a medium bowl. Add additional milk 1 teaspoon at a time until thick but pourable consistency is achieved.
14 - Drizzle the chocolate glaze generously over the warm rolls. Serve immediately while still warm for optimal texture and flavor.