Combine 20g bread flour and 100g milk in a saucepot to make tanzhong. Cover and set aside.
In a bowl, mix together 340g bread flour, 57g melted unsalted butter, 80g warm milk, 1 whole egg, 1 egg yolk, 50g sugar, 7g yeast, and ½ tsp salt (mix salt to flour first).
Add the tanzhong paste and mix together for about 8-10 minutes.
For the filling, mix 1 tbsp cinnamon powder, ⅓ cup unsalted butter, and ⅔ cup brown sugar.
Proof the dough for 1 hour before rolling and adding the filling.
Proof again for 30 minutes.
Bake at 180°C for 15-18 minutes.
