Mix flour, sugar, salt, and yeast in a bowl.
Add milk, egg, and butter, then knead the dough.
Let the dough rise for the first fermentation.
Shape the fermented dough into balls.
Prepare Soboro topping by mixing flour, sugar, butter, and salt.
Place the Soboro topping on the dough.
Allow the dough to rise for the second fermentation.
Bake in a preheated oven at 180°C for 15 minutes.