Instructions

1. Cut 4 tablespoons unsalted butter into 4 pieces and let sit at room temperature until softened.
2. Place 1/2 cup whole or 2% milk and 1/4 cup bread flour in a small saucepan. Cook over medium heat, whisking constantly, until the mixture thickens to the consistency of mashed potatoes and is steaming, 2 to 3 minutes.
3. Scrape the mixture into a small heatproof bowl and let cool until lukewarm, about 10 minutes. Meanwhile, proof the yeast for the dough and measure out the dry ingredients.
4. Place 2/3 cup whole or 2% milk in a small microwave-safe bowl or measuring cup. Microwave until warm to the touch (105ºF to 110ºF), 30 to 40 seconds. (Alternatively, warm on the stovetop and pour into a small bowl.) Sprinkle with 1 (1/4-ounce) packet active dry yeast, stir to combine, and let sit until foamy, 5 to 10 minutes. Place 2 2/3 cups bread flour, 1/4 cup granulated sugar, and 3/4 teaspoon kosher salt in the bowl of a stand mixer and whisk to combine.
5. Add the tangzhong, yeast mixture, and 1 of the large eggs to the flour mixture. Mix with the dough hook on the lowest speed until a shaggy dough forms, about 1 1/2 minutes. Increase the speed to medium and add the butter 1 piece at a time, waiting until each piece is completely incorporated before adding the next, about 5 minutes total.
6. Increase the speed to medium-high and mix for 5 minutes more. At this point, the dough should clear the sides of the bowl but stick a little to the bottom, and will be smooth, tacky, and shiny.
7. Lightly coat a large bowl with cooking spray. Scrape the dough into the bowl and flip to coat. Cover with plastic wrap or a kitchen towel. Let rise in a warm place until roughly doubled in bulk, about 1 1/2 hours, or refrigerate for at least 8 or up to 24 hours.
8. Line a baking sheet with parchment paper or silicone baking mat. Divide the dough into 15 pieces (about 50 grams each). Form each piece into a round ball by pushing down with the heel of your hand (lightly flour your hand but not the work surface if the dough is sticky) while moving your hand in a circular motion, then cupping the ball toward the end (here are step-by-step photos). Place on the baking sheet, spacing them evenly apart.
9. Lightly cover with plastic wrap or a damp kitchen towel and let rise in a warm place until puffed and about 2 1/2-inches wide, about 1 hour.
10. About 30 minutes before the buns are ready, arrange a rack in the middle of the oven and heat the oven to 375ºF. Place the remaining 1 large egg and 1/2 teaspoon water in a small bowl and whisk with a fork until broken up.
11. Uncover the buns and brush with a thin, even layer of the egg. Sprinkle with 1 teaspoon white or black sesame seeds (or a combination) if desired.
12. Bake until golden brown, about 20 minutes. Let cool for at least 5 minutes before serving, or let cool to room temperature.