Combine all the dough ingredients including the cooled Tangzhong, except for the butter in bowl of stand mixer with dough hook attached.
Mix on low until dough comes together, cover and let sit for 20 minutes.
Add 1 tbsp of soft butter and mix until butter has been fully incorporated.
Add each additional tbsp of butter after the previous has been fully worked into the dough.
Mix dough on low until dough is smooth and sticky. If you pinch a piece of the dough and pull up, it will stretch and hold.
Let dough rest and rise in a lightly greased covered bowl in a warm spot for 1-2 hours. Dough will not rise significantly, but it does expand. It can be helpful to take a photo right before proofing as many times I have felt like the dough hasn't risen but it has.
Lightly grease an 8-9 inch pan. It can be circle or square (I usually use a springform pan) and set aside.
Set dough on a lightly floured counter and divide into 8-12 equal portions depending on what type of pan you use.
Flatten slightly each portion of dough and add chocolate to center. Pinch together dough to create a ball. **See note on chocolate**
Place dough seam side down, cupping hand over it on work space and using circular motion create tension in the dough.
Place dough into prepared pan, cover and let rise for another 45 minutes in a warm area.
Preheat oven to 350°F
Bake buns for 25 minutes on middle rack. A thermometer inserted into the center of the dough should read 190°F.
Let buns cool in pan for 10 minutes, then remove and place on a wire rack. While the buns are cooling, prepare the glaze.