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 mixture. Stir with a wooden spoon until a sticky, shaggy dough comes together.
03 - Turn dough onto a lightly floured surface and knead by hand for 8-10 minutes, or use a stand mixer with dough hook. Continue until smooth, elastic, and slightly tacky but not sticky.
04 - Place dough in a greased bowl, turning once to coat all sides. Cover with plastic wrap or a clean towel. Let rise in a warm, draft-free area until doubled in volume, approximately 1 hour.
05 - Punch down 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 using a pastry brush or offset spatula.
07 - Mix brown sugar, cocoa powder, and ground cinnamon in a small bowl until well combined.
08 - Sprinkle the sugar-cocoa mixture evenly over the buttered dough. Scatter dark chocolate chips across the surface, pressing lightly to adhere.
09 - Starting from one long edge, roll dough tightly into a cylinder. Pinch the seam firmly to seal.
10 - Using unflavored dental floss or a sharp knife, cut the log into 12 equal pieces, approximately 1 1/4 inches thick.
11 - Place rolls cut-side up in a greased 9x13-inch baking dish, leaving space between each. Cover and let rise until puffy and noticeably expanded, 30-45 minutes.
12 - Set oven to 350°F and allow to fully preheat while rolls complete their final rise.
13 - Bake rolls for 22-25 minutes until set, springy to the touch, and lightly browned. The center should register approximately 190°F when fully cooked.
14 - While rolls cool slightly, whisk powdered sugar, cocoa powder, 2 tablespoons milk, and vanilla extract in a small bowl. Add additional milk 1 teaspoon at a time until desired drizzling consistency is reached.
15 - Drizzle chocolate glaze generously over warm rolls. Serve immediately while still slightly warm for optimal texture and flavor.