Bo Lo Bao is another name for Hong Kong pineapple buns. These buns are soft, fluffy, and slightly sweet, and topped with crunchy cookie topping, which resembles the texture of a pineapple. I'm also sharing three different filling options.
Combine all the ingredients in a mixing bowl. You will get a dough-like consistency. Divide into 8 portions and roll into round balls. Keep them covered in an air-tight container and keep in the fridge if you prepare this few days before. Simply let them come to a room temperature to soften a bit before wrapping
Prepare tangzhong:
Place milk and the bread flour in a small saucepan. Whisk until there is no more lumps. Cook this mixture over low-medium heat until it reaches 65 C (149 F). Remove from the heat and cover with a plastic wrap, touching the tangzhong so the skin won't form. I highly suggest using this the next day instead of using it on the same day. Store this in the fridge
Prepare the dough:
If you keep tangzhong in the fridge, take it out to let it sit at room temperature for about 30 minutes or so. Mix all the dry ingredients together in a mixing bowl. Make a well in the center and pour in the wet ingredients and the tang zhong you made earlier. Use a dough hook attachment to knead the dough on speed 2 until all the crumbly dough starts to come together into one mass, it may take about 2-3 minutes for this to happen. Don't be tempted to add any liquid. You may need to stop halfway and scrape the dough off the hook and the sides of the bowl and knead again for another 3 minutes
Gradually add in the softened butter bit by bit. The dough will turn into a REAL sticky mess again because of the butter, just keep kneading and stop halfway and scrape the dough off the hook and bowl again and continue to knead until the butter is absorbed by the dough. IT WILL HAPPEN, don't worry! DO NOT be tempted to add any extra flour. You will get a shiny smooth dough that is very elastic. This may take about 10-15 minutes on speed 4. The dough must pass a windowpane test, meaning, when you stretch a small amount of dough thinly, it won't break and a light can pass through
Place it in a lightly oiled bowl and let it rise in a warm place for 1 hour or longer depending on the temperature. It will double in size. In wintertime, I use my oven bread proof function to do this.
Prepare the cookie crust (can be done the day before too):
Combine the flour, sugar, salt, baking powder, and baking soda in a mixing bowl and set aside. Add softened butter and mix with the dry ingredients into crumbles. Add the yolk and vanilla and mix into a moist cookie dough but not too sticky. If the dough seems dry, you can add a splash of milk, about 1/2 teaspoon at a time to get into a moist cookie dough. If the dough is a bit sticky, cover the dough with a cling wrap and refrigerate for about 30 minutes to help it firm up and easier to work with
Divide into 8 portions. Work with one at a time. Use your hand to knead the dough briefly and you can feel that the dough will soften slightly. Place the cookie dough in between two cling wraps. Use a rolling pin to roll it out into a 4-inch circle
Cover with a cling wrap and refrigerate while you work on the dough
Shape the dough and 2nd proofing:
Once the dough has doubled in size, punch the dough down and then roll it into a long log and cut into 8 large pieces. Stretch the edge of each piece and tuck it back into the middle to form a smooth round ball.
If you want to fill the buns with custard, flatten it into about 4-5 inch circle and place the filling in the middle and gather the sides to enclose the buns. Roll it into a smooth round ball. Placed on a parchment paper (seam side down) about 2-inch apart. Let them proof again for another 40 minutes or until they double in size again
Put the bun + cookie crust together:
Preheat oven to 390 F (200 C) 15 minutes before the end of 2nd proofing for the bun, get the cookie dough out from the fridge.
Mix the egg yolk with water for egg wash.
Brush the surface of the bun with some egg yolk wash. Put the cookie dough gently on top of the proofed buns. Repeat with the rest of the cookie dough and proofed dough. Gently brush it on top of the cookie dough. Repeat with the rest
Place in the oven 3rd rack from the top and bake them for 15 minutes for large buns or until internal temperature is at least 190 F (88 C). Remove from the oven and let them cool down a little bit before serving. They are best served warm and can be kept up to 5 days at room temperature
Notes
If you use regular granulated sugar, put it in a food processor and process until it's fine in texture, but not into powdered sugar/icing sugar