For the brisket, put the coriander seeds, cumin seeds, mustard seeds and peppercorns in a frying pan and toast over a medium heat for a few minutes, taking care not to burn them. Allow to cool.
Once cooled, add the spices to a pestle and mortar and crush to a powder. Add the sugar, smoked paprika and cayenne pepper and mix together.
Score the inside of the beef with a sharp knife and rub the spice mix all over the joint, inside and out. Roll the joint up and tie securely in several places using kitchen string. Cover with cling film and place in the fridge overnight.
The next day, preheat the oven to 160C/325F/Gas 3. Place the brisket on a wire rack inside a roasting tray and pour the beef stock around the outside. Cover the whole tray with aluminium foil to prevent any moisture escaping. Place in the oven to cook slowly for 4-6 hours, occasionally checking that the liquid hasn’t evaporated. Add some water if it looks low.
After 4-6 hours the beef should be soft and tender. Remove from the oven and leave to rest in the aluminium foil for 25 minutes. Reserve any cooking juices.
For the milk buns, place the flour in the bowl of a freestanding mixer with a dough hook attached. Add the yeast to one side of the bowl and the salt and sugar to the other.
Warm the milk and butter together over a low heat to body temperature, the butter should just melt. Add the warm mixture to the flour and bring together using the dough hook. Knead in the machine for a further 5-8 minutes until you have a silky, shiny dough. Turn the machine off, cover the bowl with cling film and put the bowl in a warm place to prove until doubled in size - around 1-2 hours.
Once doubled in size, knock the air out of the dough and knead briefly by hand. Lightly dust a work surface with flour, split the dough into 12 equal pieces, weighing approximately 80g/3oz each, and roll into balls.
Grease a large baking tray or two small baking trays with oil. Place the balls of dough on the tray, slightly spaced apart. Cover with oiled cling film and leave to prove for an hour, or until they have doubled in size. As they prove, the rolls will touch each other.
Preheat the oven to 220C/425F/Gas 7. Discard the cling film and brush the top of the rolls with the beaten egg. Sprinkle with sesame seeds and the flaky sea salt. Bake for 8-12 minutes, or until golden-brown. Remove from the oven and leave to cool.
For the barbecue sauce, put a very large pan over a high heat and add the vinegar. Bring to the boil, then turn down the heat and slowly reduce the volume of liquid by half. Add the sugar, bourbon, cola, ketchup and juices and Worcestershire sauce. Skim off the fat from the surface of the beef brisket cooking juices and add the remaining juices to the pan. Bring back to the boil, then turn the heat down and simmer very slowly for 15-20 minutes.
Meanwhile, remove the brisket from the rack and pull the meat apart using a fork. Add the shredded brisket to the simmering sauce along with any juices left in the roasting tray. Bring back to a very low simmer and cook for a further 10-15 minutes. Turn the heat off and put the pan to one side.
Serve the beef together with the milk buns and some coleslaw to make the ultimate sandwich.