Combine Tangzhong ingredients in a pot and heat until it turns into a paste.
Mix 120g whole milk with 9g of yeast. Sit for several minutes.
320g bread flour, 35g sugar, 7g salt, and mix.
Add the milk mixture to the flour mixture and mix.
Add the Tangzhong and 1 egg and 1 yolk to the milk and flour mixture when it is well incorporated. Mix.
42g of soft butter and knead until the dough becomes less sticky. This might take up to 30 minutes by hand, or 8 minutes using a stand mixer.
Shape the dough into a ball roughly, then placed into a greased bowl. Cover for 1.5 hours or until doubled.
Take out the dough and deflate. Lightly flour a surface and then divide the dough evenly into 6 pieces.
Shape each portion into a bun or ball shape by tucking in the bottoms.
Place them on a baking tray and cover. Let rise for another 1.5 hours or until doubled.
Brush with egg wash.
Bake at about 180C for 16 to 18 minutes until golden brown. Temperature depends on oven (mine is a toaster oven, so a conventional oven might require 200C for the same amount of time).
Notes
Yes, this is almost exact to Joshua Weissman's recipe.