Cantonese Pineapple Buns (Bolo Bao) with Butter Slab

Make the tangzhong first. Whisk 20 g of bread flour with 100 ml of cold water in a small saucepan until fully combined with no lumps. Cook over medium-low heat, stirring constantly with a wooden spoon or spatula, until the mixture thickens into a smooth, glossy paste that holds lines when you drag the spoon through it — about 3–4 minutes. It should reach around 65°C if you have a thermometer. Scrape into a small bowl and cover with plastic wrap pressed directly on the surface to prevent a skin. Cool completely to room temperature before using. This can be made up to a day ahead and refrigerated.

In the bowl of a stand mixer fitted with the dough hook, combine the bread flour, instant yeast, caster sugar, and salt — keep the yeast and salt on opposite sides of the bowl. Add the cooled tangzhong, whole milk, and egg. Mix on low speed for 2 minutes until a shaggy dough forms, then increase to medium speed and knead for 8 minutes until the dough is smooth and pulls cleanly away from the bowl sides. The dough will be soft and slightly tacky — that is correct.

Add the 40 g of softened butter in four or five small pieces, waiting for each piece to be almost fully incorporated before adding the next. This takes patience — the dough will look greasy and broken at first, then come back together. Once all the butter is in, increase to medium-high speed and knead for a further 5–8 minutes until the dough is silky, elastic, and passes the windowpane test: a small piece stretched thin between your fingers should become translucent without tearing.

Shape the dough into a ball, place it in a lightly oiled bowl, and cover with plastic wrap. Leave to rise at room temperature until doubled in size, about 60–75 minutes depending on the warmth of your kitchen. While the dough rises, make the crust topping.

Make the crust topping. Cut the cold butter into small cubes. In a bowl, combine the cake flour, icing sugar, and milk powder. Add the cold butter cubes and rub quickly into the flour mixture using your fingertips — work fast to keep the butter cold — until the mixture resembles coarse crumbs with no large lumps. Add the egg yolk and mix with a fork, then with your hands, pressing the dough together until it forms a smooth, firm mass. If it feels too soft or greasy, refrigerate for 15 minutes before using. The crust dough should be firm enough to roll without sticking, soft enough to shape without cracking.

Line two baking sheets with parchment paper. Punch down the risen dough and weigh it, then divide into 12 equal pieces (roughly 55–60 g each). Working with one piece at a time, fold the edges under and roll firmly against the work surface with a cupped hand to form a tight, smooth sphere. Place on the prepared baking sheets, spacing at least 6 cm apart. Cover loosely with plastic wrap or a damp cloth and leave to proof for 45–55 minutes until noticeably puffed and the dough springs back slowly when gently pressed with a fingertip.

Divide the crust dough into 12 equal pieces. Roll each piece into a ball, then flatten between two sheets of parchment paper using a rolling pin or the base of a flat-bottomed glass into a disc roughly 7 cm in diameter — slightly smaller than the bun tops. If the crust is soft and sticking, refrigerate the discs on a tray for 10–15 minutes until firm.

Preheat the oven to 190°C / 375°F (170°C fan). Gently lay one crust disc on top of each proofed bun, centring it and pressing lightly so it adheres. It should cover roughly the top two-thirds of the bun but not drape down the sides. Using a sharp knife or a dough scraper, score the crust in a 4×4 cross-hatch pattern — four parallel lines one way, four the other — to create the signature pineapple-skin texture. Score firmly but not deeply; you are marking the crust, not cutting into the bread.

Brush the exposed bun sides (not the crust) very lightly with egg wash. This gives the bun a shine without browning the crust unevenly. Bake for 13–15 minutes until the crust is deep golden — not pale, not dark brown — and an instant-read thermometer inserted into the centre of a bun reads around 88°C. The crust should be set and just starting to crack at the score lines. Remove from the oven and transfer to a wire rack.

Eat warm. For bo lo yau, split each hot bun horizontally with a serrated knife while it is still warm from the oven. Tuck a thick cold slice of unsalted butter — about 5 g — into the opening. Press gently closed and hold for 30 seconds while the butter begins to soften against the heat of the crumb. The butter should be melting at the edges but still cold and slightly firm at the centre. Eat immediately.