Toast coriander seeds, cumin seeds, mustard seeds, and peppercorns in a pan until fragrant.
Crush cooled spices to a powder and mix with sugar, paprika, and cayenne.
Score beef, rub with spice mix, roll, tie with string, and refrigerate overnight.
Preheat oven to 160°C and slow-cook brisket in a roasting tray with stock for 4-6 hours.
Rest beef for 25 minutes and save the cooking juices for the sauce.
Combine flour, yeast, salt, and sugar in a mixer bowl.
Add warmed milk and butter to flour, then knead for 5-8 minutes until silky.
Cover and prove dough in a warm place until doubled, then divide into twelve 80g pieces.
Roll into balls, place on oiled trays, and prove again until doubled.
Glaze buns with egg, sprinkle with sesame and salt, and bake at 220°C for 8-12 minutes.
Reduce vinegar by half, then simmer with sugar, bourbon, cola, ketchup, and juices for 15 minutes.
Pull brisket apart with a fork and simmer the shredded meat in the sauce for 10-15 minutes.
Assemble the sandwiches by serving the pulled beef inside the fresh milk buns.