Step 1: Make the Tangzhong
In a small saucepan, whisk together 3 tbsp bread flour, ¼ cup milk, and ¼ cup water.
Cook over medium heat, whisking constantly, until it thickens into a smooth paste (2–3 minutes).
Remove from heat and let cool.

Step 2: Prepare the Dough
In a large bowl or stand mixer, combine bread flour, sugar, salt, and yeast.
Add warm milk, egg, and the cooled tangzhong.
Mix on low until a dough forms.
Add softened butter a little at a time, kneading until incorporated.
Knead for 10–12 minutes until smooth and elastic. The dough will be soft but not sticky.

Step 3: First Rise
Form dough into a ball and place in a greased bowl. Cover and let rise in a warm spot for 1–1.5 hours, or until doubled in size.

Step 4: Make the Chocolate Paste
While the dough is rising, combine cocoa powder, sugar, milk, and melted butter in a small bowl. Stir until smooth. Set aside.

Step 5: Shape the Bread
Punch down the dough and roll it out into a large rectangle (about 8×14 inches).
Spread the chocolate paste evenly across the surface, leaving a small border around the edges.
Roll the dough up tightly like a jelly roll, starting from the short end.
Place seam-side down in a greased 9×5 loaf pan.

Step 6: Second Rise
Cover loosely and let rise for 45–60 minutes, or until the loaf is puffy and has risen about 1 inch above the rim of the pan.

Step 7: Bake the Bread
Preheat oven to 350°F (175°C).
Brush the top of the loaf with milk or a beaten egg for a golden finish.
Bake for 30–35 minutes until the top is deep golden brown and the loaf sounds hollow when tapped.
If browning too quickly, cover with foil during the last 10 minutes.

Step 8: Cool & Slice
Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Slice with a sharp bread knife to reveal the swirl.