Grab the bun from the steamer and flop it open on a plate. Slather the inside with the hoisin sauce, using a pastry brush or the back of a spoon. Arrange the pickles on one side of the fold in the bun and the slices of pork belly on the other. Scatter the belly and pickles with sliced scallion, fold closed, and voila; pork bun. Serve with sriracha.