Bo lo bao (菠萝包, literally pineapple bun — despite containing no pineapple, the name refers to the crackled, golden top crust that resembles a pineapple's skin pattern) is the most iconic product of the Hong Kong Chinese bakery (bing jia, 饼家) tradition. A soft, slightly sweet yeasted bun with a crisp, buttery, crackled sugar topping, it is eaten in the morning with butter (a thick slab of butter inserted into a split hot bun — the butter melts slightly from the warmth of the bun), with hot milk tea (nai cha, 奶茶), or at any time as a snack. It is an emblem of Hong Kong's hybrid food culture — a Chinese bun with a Western pastry-style topping.
The bun dough: A standard enriched yeast dough — plain flour, yeast, sugar, butter or lard, milk or water, eggs. The dough should be soft, slightly rich, and produce a bun that is very soft and fluffy without being too sweet. The topping (the defining element): 50g cake flour (low-protein), 50g icing sugar, 40g cold butter (cubed), 1/2 egg yolk, 1/2 tsp vanilla extract, 1/4 tsp baking powder. Rub the butter into the flour and sugar until crumbly. Add the egg yolk and vanilla. Press into a dough. This topping dough is shaped into a flat disc and placed on top of the shaped bun before the second rise. As the bun bakes, the topping cracks into the characteristic pineapple-skin pattern. Baking: 190C for 12-15 minutes until the bun is golden and the topping is crackled.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009); Ken Hom, Complete Chinese Cookbook (2011)