Make tangzhong by whisking 40g (⅓ cup) bread flour and 177g (¾ cup) water in a small saucepan until combined. Set over medium-high heat and cook, whisking constantly, until thickened to a runny paste, 1–2 minutes. Transfer tangzhong to a small bowl and chill, uncovered, until cold, at least 2 hours
In the bowl of your stand mixer, whisk milk powder, salt, yeast, and the rest of the bread flour until combined. In a separate bowl, add one egg, the milk and honey and mix until combined. Add the wet ingredients to the bowl of the stand mixer along with the tangzhong and butter.
Using the dough hook attachment, mix on low speed until a shaggy dough comes together, around 14 minutes. Increase the speed to medium and once the dough springs back when you poke it, around 9 minutes, turn mixer off.
Lightly coat a large bowl with butter or cooking spray. Transfer dough to bowl and lightly coat top of dough with nonstick spray. Cover with cling film and chill dough at least 4 hours and up to 1 day.
Prepare the filling and icing while the dough is proofing