Preheat the oven to 220°C.
For the mayonnaise, mix the egg yolk with the vinegar and mustard in a bowl. Ever so slowly, trickle in the oil, whisking all the while, until you have a smooth, thick mayonnaise. Season well with salt and pepper and add a good squeeze of lemon juice.
Place the pork in a deep saucepan, cover with water and add a couple of pinches of salt. Bring to the boil and cook for 15 minutes, then remove and pat dry with kitchen paper. Transfer to a roasting tray and add the vinegar and a good drizzle of vegetable oil. Sprinkle a good pinch of sea salt over, then rub everything into the scored skin. Roast for 30–45 minutes, keeping an eye on it, until the skin is uniformly golden and crisp and the roasting juices run clear when you pierce the meat with a sharp knife. Remove from the oven and leave to cool.
For the poached rhubarb, bring the cider, brown sugar and cinnamon stick to the boil in a non-reactive saucepan. Turn down the heat to low, drop in the rhubarb and poach for 10 minutes, or until tender but not falling apart. Leave to cool.
When you’re ready to eat, mix the mayonnaise with the paprika and lemon juice in a small bowl. Slice the pork belly into 1cm thick slices that will fit into the baps and fry in a skillet or heavy-based frying pan with a very small amount of vegetable oil on a medium heat until crisp. Fill each bap with a slice or two of pork belly, a few sticks of poached rhubarb, a small handful of salad leaves and a good drizzle of the smoky mayo.