Mix together yeast and warm milk and set aside for 5 minutes. The milk should feel comfortably warm to the touch, but not hot, otherwise it will kill your yeast.
Combine flour, egg, milk and yeast mixture, 1/2 cup of tangzhong starter in a medium mixing bowl. Mix together with your hands until combined. Cover with a damp kitchen or paper towel and let sit for 20 – 30 minutes.
Pour dough out onto clean countertop or nonstick surface. Don't worry if it is quite sticky. Gently smooth out the dough and sprinkle surface with sugar and salt. Gently fold the dough over the sugar and salt a few times. Slowly push the dough out until it is thinly smeared on the countertop, then gather back in to one lump. Repeat the smearing and gathering until you no longer feel the salt and sugar grains.
Gently smooth out the dough and place softened pieces of butter in the center. Gently fold the dough over the butter a few times. Again, slowly push the dough out until it is thinly smeared on the countertop, then gather back in to one lump. Repeat the smearing and gathering until you no longer see streaks of butter.
Knead using the slap and fold method. Do not add flour. Lift the dough up and "slap" it onto the countertop. Fold up, raise, and repeat until the dough is smooth and passes the window pane test. You'll notice the dough sticking more to itself than to your hands or the countertop. Shape into a ball and place into a lightly greased mixing bowl. Cover with plastic wrap and let rise for 40 – 60 minutes until roughly doubled in size.
Grease your baking pan or loaf tin. Remove the dough from the bowl onto a lightly floured surface. Use your rolling pin to roll into an 8'' x 12'' rectangle, with the longer side parallel to you. Evenly spread the red bean paste across the surface of the rectangle, being sure to spread to the ends.
Roll up the dough tightly so that you have a 12'' long log. Use a knife or your bench scraper to cut the log in half. Twist the halves together so that the red bean faces up, and coil the dough into your baking pan or loaf tin. Cover with plastic wrap and allow the loaf to proof for 45 – 60 minutes. When you press your finger into the loaf, it should slowly come back about halfway. Towards the end of the proof, preheat oven to 350°F
Place in the lower third of the oven and bake for 35 minutes until golden brown and makes a hollow sound when tapped. Remove from oven and let cool for ~ 1 hour before cutting.