Instructions

1. Place 1/2 cup whole milk and 1/3 cup water in a small microwave-safe bowl. Microwave on HIGH until just warm to the touch, 25 to 35 seconds. (Alternatively, warm on the stovetop.)
2. Place 354 grams cake flour, 1/3 cup granulated sugar, 1 (1/4-ounce) packet instant yeast, and 1 teaspoon baking powder in the bowl of a stand mixer and whisk to combine. Fit the dough hook onto the mixer.
3. Turn a timer on for 10 minutes. Turn the mixer on to the lowest speed and slowly pour in the milk mixture. Check the dough after 2 minutes of mixing -- if it has not formed a ball, add more cake flour a teaspoon at a time (let it mix for a minute before adding more) until it forms one. Stop the mixer and timer and squeeze the dough after 5 minutes -- if it does not feel soft and supple and stretch easily, mix in a teaspoon of water (the dough may separate but will come back together after a minute or so). Continue timing and mixing until the 10 minutes is up. The dough should not stick to the bowl but should be smooth, soft, and supple.
4. Cover the bowl and let rise in a warm place until slightly puffed, about 1 hour. (Alternatively, cover and let rise for at least 8 and up to 24 hours in the refrigerator.) When ready, the dough should feel like a soft marshmallow.
5. Meanwhile, prepare the steam papers. Cut out 15 (3 1/2-inch) squares of parchment paper. (Alternatively, flip 15 paper muffin liners inside out and flatten, then brush with a thin layer of vegetable oil.) Place 1 tablespoon vegetable oil in a small bowl.
6. Turn the dough out onto a work surface and divide into 15 portions (about 1 1/2 ounces or 42 grams each). Form each portion into a smooth ball, using floured hands if the dough is sticky.
7. On a lightly floured work surface, roll out a dough ball into a 2 3/4 x 5-inch oval that’s a scant 1/4-inch thick. Brush with a thin layer of vegetable oil, then fold in half crosswise. Place on a parchment square or muffin liner. Place on a baking sheet. Repeat with the remaining dough balls, the paper can overlap slightly on the baking sheet but the buns should not touch.
8. Cover the buns loosely with plastic wrap or a damp kitchen towel and let rise in a warm place for 30 minutes (they may puff a little but will not rise much).
9. A few minutes before the buns are ready, prepare a steamer for steaming and bring the water to a boil over medium heat. Place as many buns in the steamer that can fit in a single layer without touching, although the papers can overlap slightly (we fit 5 in a 12-inch steamer). Cover and steam over medium heat (you want gentle steam) until the dough is cooked through, about 10 minutes. Meanwhile, place the unsteamed buns in the refrigerator.
10. Transfer the steamed buns to a plate. Steam the remaining buns, adding more water to the steamer between rounds if needed. Serve warm.