Grind all the ingredients together to a fine powder in a spice grinder or mortar and pestle.
Char siu
Rinse the pork and pat dry with a paper towel. Cut into 1½"-thick slices and place in a large bowl.
Mix the remaining (marinade) ingredients together in a small bowl. Adjust for sweetness by adding more hoisin or soy sauce as you like. (Char siu is generally a bit sweet.) Pour ⅔ of the marinade over the pork, setting the rest aside for basting later. Stir well so that the meat is well coated. Cover with plastic wrap and refrigerate overnight, stirring it a few times if you can.
Bring the pork and reserved marinade back to room temperature (about an hour) before roasting.
Preheat over to 475°F.
Lay out the marinated pork - spaced a few inches apart - on a roasting rack placed over a foil-lined baking pan. Roast the pork in the upper part of the oven for about 30-40 minutes. Baste the meat every 10 minutes or so by using tongs to dip it in the reserved marinade and placing it back on the rack, turned over.
When the meat is done (it should be browned and slightly charred), remove from oven and let rest for 10-20 minutes.
Refrigerate or wrap in plastic wrap and freeze.
Steamed buns
Put the yeast in a small bowl and add the water. After about a minute, whisk in the oil and set aside.
Mix the flour, sugar, and baking powder in a bowl and pour in the yeast water. Stir with a wooden spoon until all the flour has been mixed in to form a ragged lump. Add more lukewarm water by the teaspoon if needed.
Use your hands to form the dough into a ball and transfer to a clean counter top. Knead the dough for about 5 minutes or so until it is soft and smooth. The dough should be elastic (it should spring back slowly when poked with a finger) and not sticky.
Transfer the dough to a lightly-oiled bowl, cover with plastic wrap and place in a warm area (e.g., in the oven or microwave with light on). Allow the dough to rise, about 45 minutes.
Meanwhile, cut parchment paper into 24, 3" squares and lay out on a baking sheet or other work area.
Once the dough has roughly doubled in size, move it to a lightly-floured surface and shape into a thick disk. Cut in half and put one piece back into the bowl and cover with wrap.
Roll the other half into a 12" log and cut into 12 pieces. Cover with a kitchen towel while you work on each piece. Roll each piece into a ball and flatten into a disk. Then, using a rolling pin, roll into a circle of 3-3½" diameter (about a ¼" thick.) Place on a square of the parchment paper and cover with a towel. Repeat with the other pieces and remaining half disk.
Lightly brush one side of each circle with canola oil and fold over to make a semicircle. Cover and and allow to rise again for another 30 minutes.
In the mean time, get the steamer ready. When the final rise is over, place the parchment squares directly onto the steamer tray. Don't overcrowd. Steam for 7-8 minutes until the buns are puffed up.
At this point you can wrap the buns in their parchment squares and freeze them if you like. To reheat, just steam them for a few minutes.
Assembling the sandwich
To assemble, thinly slice the roasted pork and stuff the buns. Garnish with scallions and serve.
Recipe by Oishii Rasoi at https://www.oishiirasoi.com/cantonese-style-roast-pork-buns-char-siu-bao/