Make the tangzhong: Mix the flour and water together, then cook over low heat, stirring constantly, until it thickens to a baby food-like consistency. Transfer to a bowl, cover with plastic wrap touching the surface, and refrigerate for 45 minutes.

Activate the yeast with 1/4 cup milk (50 grams) and a pinch of sugar.

Combine all ingredients (except the butter) and mix until well combined. Add the butter and knead until the dough is smooth and stretches thin. Cover and let the dough rest for 20 minutes. Divide the dough as desired (I divided into 90-gram pieces), shape into balls, and let rest for 15 minutes.

Roll each dough ball into a thin, long oval. Fold the edges to shape like Vietnamese baguettes. Proof at 86°F (30°C) for 30 minutes, or until doubled in size. Preheat the oven to 355°F (180°C). While the oven heats, brush the tops with the egg wash, score the bread, pipe the butter mixture into the cuts, sprinkle with sugar if desired, and bake at 340°F (170°C) for 20 minutes.