In the bowl of a stand mixer( or in a large bowl if kneading by hand), combine flour, , instant yeast, sugar and salt. Whisk to combine.Add water, milk and tangzhong and knead until just combined. Cover and let rest for 5 min.After resting time, start kneading on medium- low and add butter. Keep kneading on medium-high until smooth and elastic (about 10 minutes).
Divide dough into one 1/3 (300g) and one 2/3 (600g) portions.
Shape the 2/3 portion into a ball and place into a greased bowl. Cover and set aside.
Add cacao powder and milk to the 1/3 portion and knead on low until completely combined. Shape into a ball, place in a greased bowl, cover and let rest until doubled in size (1h-1h30)
Roll out plain dough into a 20x30cm rectangle.Roll out cacao dough into a 20x15cm rectangle.
Place cacao dough over one half of plain dough and fold the remaining plain dough over. Roll out to into a 20×30 cm rectangle again.
Cut dough cross wise and stack the two doughs. Roll out to combine.Repeat two more times.
You should end up with a 20×30 cm dough made of several layers.
Cut dough into 3 equal stripes, lengthwise.
Roll out each stripe into a rope and braid the 3 ropes together.Fold ends under so that the bread fits the pan
Let rise until the bread is 3 times the size (about 45-60 min).
Preheat oven to 175C degrees and bake for 30 minutes. Let cool down into the pan for 15 min then cool completely on a rack.