Ingredients
85 grams (1/3 cup plus 1 tablespoon) water
322 grams (2 1/4 cups plus 2 1/2 tablespoons) bread flour, divided
102 grams (1/2 cup) warm milk (100 to 110°F)
1 large egg
50 grams (4 tablespoons) sugar
6 grams (1 teaspoon) kosher salt
6 grams (3 teaspoons) active dry yeast
30 grams (2 tablespoons) butter, at room temperature, plus more for greasing
77 grams (1/3 cup) salted butter, at room temperature
78 grams (4 tablespoons) chunky peanut butter
78 grams (1/3 cup) brown sugar
1 large egg yolk
4 grams (1 teaspoon) vanilla extract
187 grams (1 1/2 cups) all-purpose flour
5 grams (1 teaspoon) baking powder
8 grams (1 tablespoon) cornstarch
1/4 teaspoon kosher salt
17 grams (1 tablespoon) dark corn syrup
Assembly
237 grams (1 cup) water, room temperature
8 grams (1 tablespoon) cornstarch
430 grams 1 (14.3-ounce can) sweetened, mashed red beans
Ingredients
  1. Whisk the water and 2 1/2 tablespoons bread flour together, in a microwave-safe bowl, until no lumps remain. Microwave the tangzhong for 1 minute, in 30-second intervals, or until the mixture is a thick slurry.
  2. Add the tangzhong and warm milk to a stand mixer fitted with a whisk attachment. Mix on medium-low speed until just combined, then add the egg and whisk until homogenous. Swap the whisk attachment for the dough hook, and add the remaining bread flour, sugar, salt, and yeast. Mix the dough on medium-speed until smooth and barely sticking to the bottom of the bowl, about 5 minutes. Add the butter, stopping to scrape down the sides of the bowl as necessary. Once the butter is incorporated, knead another 5 minutes on medium speed.
  3. Lightly grease a medium bowl with butter. Once the dough is fully mixed—the dough should still be slightly sticky to the touch—form it into a ball, and place in the oiled bowl. Cover with a large plate or kitchen towel and let rest in a warm place until doubled in size, about 1 hour.
  4. Meanwhile, make the peanut streusel: Change the dough hook for the paddle attachment, and cream the butter and peanut butter on medium speed until smooth, about 1 minute. Add the sugar, egg yolk, and vanilla, and mix until just incorporated, then the all-purpose flour, baking powder, cornstarch, salt, and syrup. Mix just until the dough starts to clump, forming crumbly bits around the size of peas. (Clump the streusel with your hands to form clusters of varying sizes). Cover and set aside at room temperature until ready to top the buns.
  5. Line 2 baking sheets with parchment paper and whisk together the remaining cup of water with the cornstarch in a small bowl until smooth. Set the prepared sheets and slurry aside.
  6. Divide the rested dough into 12 balls (each weighing 50 grams). Working with one portion at a time, stretch the ball into 3-inch circle. Spoon 2 tablespoons (about 35 grams) of red bean paste into the center, and pinch the dough around the filling until entirely enclosed. Flip and roll the ball, seam-side-down, around on the work surface to seal the dough around the filling. Be careful not to stretch the dough too tightly around the filling, or it will leak.
  7. Dip the smooth side of the filled bun into the cornstarch slurry and place onto a lined sheet (6 filled buns, spaced at least 2 1/2 inches apart, should fit on each sheet, so plan accordingly). Press 2 tablespoons of the peanut streusel onto the wetted bun, flattening it slightly to a 3-inch disc. Repeat with the remaining dough, filling, and topping. Loosely cover both trays with a kitchen towel, and let the buns rise in a warm place until doubled again in size, about 1 hour. Meanwhile, heat the oven to 375°F,
  8. Bake the buns until golden brown, rotating halfway, about 12-14 minutes total. Cool the soboro-ppang on a rack for 10 minutes before digging into. Enjoy warm or at room temperature, with a cup of coffee or a glass of cold milk! The buns can be stored in an airtight container on the counter for up to 3 days.