01 - Combine flour, cocoa powder, yeast, sugar, and salt in a large mixing bowl. Whisk thoroughly to distribute the cocoa and yeast evenly throughout the dry ingredients.
02 - Pour in warm milk (105-110°F), melted butter, and the egg. Mix with a wooden spoon or stand mixer fitted with the dough hook until a sticky, shaggy dough comes together.
03 - Knead the dough on a lightly floured surface for 8-10 minutes by hand, or 5-6 minutes with a stand mixer on medium-low speed. The dough should become smooth, elastic, and slightly tacky but not sticky.
04 - Place the dough in a greased bowl, turning once to coat all surfaces. Cover with plastic wrap or a damp towel and set in a warm, draft-free area. Let rise until doubled in volume, approximately 1 hour.
05 - Punch down the risen dough to release air. Turn onto a floured surface and roll into a 16x12-inch rectangle, maintaining even thickness throughout.
06 - Spread the softened butter evenly across the entire surface of the rolled dough, leaving a 1/2-inch border along the edges.
07 - Combine brown sugar, cocoa powder, and cinnamon in a small bowl. Mix thoroughly to break up any lumps and integrate the spices.
08 - Sprinkle the sugar-cocoa mixture evenly over the buttered dough. Distribute dark chocolate chips across the surface, pressing lightly to adhere.
09 - Starting from a long edge, roll the dough tightly into a log. Use a sharp knife or unflavored dental floss to cut the log into 12 equal rolls, approximately 1 1/2 inches thick.
10 - Arrange rolls cut-side up in a greased 9x13-inch baking dish, leaving space for expansion. Cover and let rise in a warm place until puffy and noticeably increased in size, 30-45 minutes.
11 - Preheat oven to 350°F convection or conventional setting.
12 - Bake rolls for 22-25 minutes until set and lightly browned. The tops should spring back when gently pressed, and an instant thermometer inserted into the center roll should read 190-200°F.
13 - While rolls cool for 5-10 minutes, whisk powdered sugar, cocoa powder, 2 tablespoons milk, and vanilla extract in a medium bowl. Add additional milk 1 teaspoon at a time until reaching a thick but pourable consistency.
14 - Drizzle the warm chocolate glaze generously over the slightly cooled rolls. Serve immediately while still warm for optimal texture and flavor.