In a bowl, combine the bread flour, all-purpose flour, baking powder, sugar, and salt. Mix lightly by hand, then make a well in the center and add the yeast.

Add the beaten egg yolk and milk, and start mixing.

Knead the dough in the bowl. Once it starts to come together, add the butter and mix further, then transfer to a work surface.

Press the dough with the heel of your hand.

Fold the dough. Repeat steps 5 and 6 until the dough is no longer sticky.

Pinch the edge of the dough.

Lift and slap the dough onto the work surface.

Fold the dough. Repeat steps 7, 8, and 9.

Continue until the dough is smooth.

Place the dough in a bowl, cover with a damp cloth, and let it rise until doubled or 2.5 times in size (about 40–50 minutes at 95°F/35°C).

Divide the dough into 8 equal pieces using a dough scraper.

Shape each piece into a smooth ball.

Cover with a damp cloth and let rest for 10–15 minutes (bench rest).

Shape the donuts: Use your finger to poke a hole in the center, lift the dough, and use both thumbs to gently stretch and widen the hole.

Make sure the center hole is large enough. Shaping the surface tightly at this stage will help create a nice white ring after frying.

Cover with a damp cloth and let rise at 95°F/35°C for 30–40 minutes, until slightly larger.

Fry in oil at 350°F (180°C) for about 1 minute per side.

Transfer to a wire rack to cool.

If coating with granulated sugar, do so while the donuts are still warm. The sugar won't stick well once they cool.