Soften butter; set aside. Warm water until 40℃ / 104 F. (Microwave at 600W for 30-50 seconds.) Crush the lumps of dry milk and make it powder-like.

Put bread flour, cake flour, and dry milk in a bowl, stir well. Make a dent in the center and add sugar and dry yeast to the dent. Add salt to the edge away from the dry yeast.

First, pour lukewarm water over the sugar and dry yeast and then pour over the flour while drawing circles. Add egg yolk. Fold roughly with a spatula and use your hand to knead until the dough almost comes together.

Put the dough in a stand mixer. I used KitchenAid Artisan 3.3L. Knead it for 2 minutes at speed 2. Next, knead for 8 minutes at speed 3-4 until smooth. In the middle of kneading, remove the dough from the hook, turn it over, and knead it evenly.

Add the softened butter. Knead at speed 2 for 2 minutes and then knead at speed 3-4 for 8 minutes until smooth and elastic. In the middle of kneading, remove the dough from the hook, turn it over, and knead it evenly.

Use your hands to stretch gently one part of the dough thinly. It's ready if it doesn't tear. Shape it into a ball and put it in a lightly oiled bowl. Cover it with cling wrap twice and leave it for 1 hour. Let it sit in a fridge, vegetable compartment, overnight.

The next day, the dough should rise double in size. Take it out of the vegetable room and leave it at room temperature for 1 hour. (When it's hot, such as in the summer, shorten the time.) It's ready if the temperature in the center of the dough reaches 15℃ / 59 F. Press it gently to punch the air out.
★ If it's doesn't reach 15℃/ 59 F even after 1 hour, after punching, divide it into four pieces, cover with a tightly wrung wet cloth and leave for 15-30 minutes until 15 ℃ / 59 F.

Divide into 20 equals, punch down, and roll to shape balls. (weight per one about 34g) Cover with a tightly wrung wet cloth and leave for 15 minutes.

Punch down again and shape them into balls. Pinch the seams to stick together. In this process, it's better to cover with a wet cloth or cling wrap to prevent the dough from drying.

Arrange them in a parchment-lined cooking tray with the seam side facing down. Let it rise at 40℃ / 104 F (or a warm place) until doubled in size for around 30-50 minutes. Preheat an oven to 200℃ / 392F.

Spray water lightly and sprinkle bread flour lightly. Lower the oven to 180℃ / 356 F and bake for 18-20 minutes until light brown. Cool on the rack. It's all done! Thank you.