Ingredients
2 1/3 cups (300 grams) all-purpose flour, plus more for dusting
1/2 cup (60 grams) cornstarch
1/2 teaspoon kosher salt
1 1/4 teaspoons (4 grams) instant yeast
3 tablespoons (38 grams) granulated sugar
1/2 cup (120 grams) whole milk
1/2 cup (120 grams) water, at room temperature
1 tablespoon tablespoon neutral oil, such as grapeseed
3 salted egg yolks (see Author Notes for recipe link)
3 tablespoons unsalted butter, room temperature
3/4 cup (90 grams) confectioners’ sugar
3 tablespoons (24 grams) milk powder
3 tablespoons (23 grams) cornstarch
2 tablespoons (32 grams) evaporated milk
1 1/2 teaspoons pure vanilla extract
1 tablespoon flaky salt, such as Maldon
Instructions
  1. Combine all of the dough ingredients in the bowl of a stand mixer fitted with a dough hook. Knead on medium-low speed until the dough looks smooth and elastic, 7 to 10 minutes. Lid the bowl with a large plate or damp towel, and let proof in the fridge for at least 8 hours and up to 24.
  2. Meanwhile, make the filling: Cook the salted yolks in a small pot of boiling water until hard-boiled, about 5 minutes. Use a slotted spoon to transfer the yolks to a stand mixer fitted with the paddle attachment and beat on medium speed until finely mashed. Add in the remaining filling ingredients and mix on medium until smooth and the texture of a fluffy buttercream, 3 to 5 minutes. Transfer filling to an airtight container, and freeze for at least 2 hours or up to overnight.
  3. The following day, scoop golf ball-sized balls (each weighing about 15 grams) of the frozen filling onto a baking sheet (you should get about 16), press a few flakes of salt onto each, and freeze until you’re ready to fill buns.
  4. Punch down the dough, and roll into a 1 1/2–inch thick rope on a lightly floured work surface. Divide the dough into 16 pieces (each weighing about 35 grams). Working with one piece at a time, turn the dough cut side up, and place a frozen ball of filling in the center. Pinch the dough around the filling, then flip the ball seam-side down. Roll around on the work surface to seal the seam. Place the filled bun onto a square of parchment, and set the parchment inside your steamer, giving each bun an inch or so of clearance. Repeat with all of the dough and filling. Lid the steamer, and let the buns rise once more, another 30 minutes. (If you have a mini steamer like me, simply let the buns rise, covered, on a sheet pan, and steam in batches.)
  5. When ready to cook, bring the water to a boil, and steam the buns for 7 minutes, or until puffed and cooked through. Remove the buns from the steamer, and let cool slightly before devouring.