Bo-lo-bao: Chinese pineapple buns

Ingredients:

Bread dough

• 2 tbsp (240 g) + 2 tbsp (18 g) bread flour + extra for sprinkling
• 1/4 cup (57 g) cold whole milk
• 4 tbsp. l. (50 gr.) sugar
• 1.5 tsp (5 g) active dry yeast
• 4.5 tbsp unsalted butter, room temperature
• 1 large egg, straight from the refrigerator
• 3 tbsp. l. (21 g.) milk powder
• 1/4 cup (28 g) cake flour
• 1 tsp (3 g) coarse salt

Cookie dough topping

• 5.5 tbsp unsalted butter, room temperature
• 5 tbsp. l. (62 gr.) sugar
• 1 large egg yolk
• 2 tbsp. l. (14 g.) milk powder
• 1/4 tsp vanilla extract or vanilla paste
• 2/3 cup (80 g) premium flour
• 0.5 tsp. (3 g.) soda
• 1/4 tsp (1 g) baking powder
• Cooking spray

Beaten egg

• 1 large egg yolk

Preparation:

1. Bread dough:
   
   In a medium saucepan, combine 2 tablespoons (18 g) bread flour with 1/2 cup cold water until smooth. Heat, stirring constantly, over medium heat until thickened and opaque, about 2 minutes. Transfer to a stand mixer and let cool slightly. Add the milk and 1 tablespoon sugar and beat by hand until smooth, or until the dough temperature drops to 104°F (40°C), about 30 seconds. Stir in the yeast until smooth. Let stand until foam appears on the surface, about 5 minutes.
2. Meanwhile, grease a large bowl with 1/2 tbsp butter and set aside.
3. Add the remaining ingredients to the yeast mixture in this order: egg, dry milk, the remaining 2 cups (240 g) bread flour, cake flour, the remaining 3 tablespoons sugar, and salt. Fit the dough hook on the mixer and mix on low speed until all the flour is moistened, about 1 minute. Increase the speed to medium and mix until the dough is smooth and passes the gluten window test, about 10 minutes (see Note). Beat in the remaining 1 tablespoon butter on medium-low speed until smooth. Repeat, adding the butter 1 tablespoon at a time, mixing after each addition, until the dough comes together, about 6 minutes.
   
   Note
   
   To test for a gluten window, slowly tear off a small piece of dough. If you can stretch it thin enough that your finger is visible on the other side, the dough is ready. If it still breaks, knead the dough at medium speed for another 2 minutes and test again.
4. Turn the dough out onto a lightly floured work surface and form it into a tight ball, tucking the edges toward the center. Place it in the prepared bowl, grease it with butter, cover with plastic wrap, and let it rise in a warm, draft-free place until doubled in size, about 90 minutes.
5. Topping:
   
   Using a mixer fitted with the paddle attachment, beat the butter and sugar at medium speed (no need to wash the bowl) until smooth but not fluffy, about 30 seconds. Stir in the egg yolk, dry milk powder, and vanilla until smooth. Sift in the flour, baking soda, and baking powder and mix on low speed until no dry lumps remain. Transfer the dough to a large sheet of plastic wrap or a resealable plastic bag and form into a 10 cm (4 in) disc. Wrap the dough in plastic wrap and refrigerate until ready to use.
6. Line 2 baking sheets with parchment paper and set aside.
7. Knead the bread dough in a bowl until it forms a firm ball. Divide into 8 equal pieces, each weighing approximately 80 grams (3 oz). Flatten 1 piece into an 8 cm (3 inch) disk and gather the edges to meet in the center. Turn over and roll into a ball on a work surface, holding the top of the dough in place with one hand until the ball is completely smooth. Repeat with the remaining pieces and place 4 balls on each prepared baking sheet. Cover each baking sheet with plastic wrap sprayed with cooking spray. Let rise in a warm place until each ball has puffed up to about 10 cm (4 inches), about 1 hour.
8. Position racks in the upper and lower thirds of the oven and preheat oven to 350°F (175°C).
9. Meanwhile, divide the cookie dough disk into 8 portions, each weighing approximately 32 grams (1 oz), and roll each portion into a ball. Place 4 balls on a large sheet of plastic wrap and cover with another piece of plastic wrap. Gently flatten each ball into a 5 cm (2 in) diameter disk, then roll it between two pieces of plastic wrap with a rolling pin until each ball forms a 10 cm (4 in) diameter circle (the circles don't have to be perfect). Place the circles on a plate, separating each piece with plastic wrap. Refrigerate until ready to assemble. Roll out the remaining cookie dough in the same way. Refrigerate on top of the first batch while the buns proof.
10. Beat the egg yolk in a small bowl until smooth and set aside. Remove the plastic wrap from the buns. Place one circle of cookie dough on each bun and brush with the beaten yolk. Using the blunt side of a small knife, draw a grid on each bun, leaving 2 cm between lines.
11. Bake, rotating the pans halfway through, until the tops of the buns are golden brown and the edges are light golden, about 15 minutes. Let cool for 10 minutes, then carefully transfer each bun to a wire rack. Serve warm or completely cooled.