Pineapple Bun (Bo Lo Bao)

Instructions

1. Make the dough: in a stand mixer bowl, combine the bread flour, sugar, instant yeast and fine salt. Add the egg and lukewarm whole milk and knead with the dough hook on low until a shaggy dough forms, 2 minutes. Add the 30 g softened unsalted butter and knead on medium until smooth, elastic and able to stretch into a thin translucent sheet, 10 to 12 minutes.
2. Shape the dough into a ball, place it in a lightly greased bowl, cover and let rise in a warm spot until doubled, 60 to 90 minutes.
3. Meanwhile, make the topping: beat the 50 g softened unsalted butter with the powdered sugar until pale, about 2 minutes. Beat in the egg yolk and 10 ml milk. Sift in the cake flour, dry milk powder, baking soda and baking powder and mix just until a soft dough forms. Wrap and refrigerate until firm enough to handle, about 30 minutes.
4. Punch down the dough, divide it into 8 equal pieces (about 72 g each) and shape each into a tight, smooth ball. Arrange them 3 inches (8 cm) apart on the parchment-lined baking sheet, cover loosely and let rise until puffy and nearly doubled, 45 to 50 minutes. Heat the oven to 375°F (190°C) about 20 minutes before the rise ends.
5. Divide the topping into 8 pieces (about 33 g). Press each between two sheets of plastic wrap into a 3-inch (8 cm) disc about 1/8 inch (3 mm) thick.
6. Peel a topping disc and lay it over each risen bun, pressing lightly so it hugs the top without deflating the bun. Brush the topping with the egg yolk wash; if you like, score a light crosshatch with the back of a knife.
7. Bake until the buns are deep golden and the topping has cracked into a checkered crust, 15 to 18 minutes. Cool on a rack for 10 minutes and serve warm.