To make the XO paste, preheat oven to 180C. Wrap the shrimp paste in a piece of foil and roast until pungent (5 minutes), then cool, coarsely crumble and set aside. Drain the dried prawns (reserve soaking water), coarsely chop the prawns and set aside. Heat the oil in a saucepan over medium-high heat, add the scallops and prawns and fry until the scallops are crisp (2-3 minutes). Add the garlic and shallots and cook, stirring occasionally until golden brown (4-5 minutes, don’t overcook or the garlic will become bitter). Add the chillies and jamon, stir occasionally until the jamon is crisp (2-3 minutes), then crumble in the roasted shrimp paste. Remove from heat, then stand to cool slightly then add the reserved prawn soaking liquid, return to the heat and simmer until the water evaporates (5-10 minutes). Set side to cool a little then blend with a stick blender until smooth. Refrigerate in a sterilised jar for up to a month.
To make the XO sauce for the cockles , combine 3 tbsp of the XO paste in a saucepan with the chicken stock, oyster sauce and sugar, bring to the boil, then add 1 tbsp of the cornflour slurry mixture and stir continuously to combine. Bring to the boil again and simmer until thick and glossy and the sauce coats the back of a wooden spoon (1-2 minutes). Set aside and keep warm.
Heat a large wok until smoking hot over high heat, add the oil, water and cockles, cover with a lid and cook, shaking wok occasionally, until the cockles open (1 minute). Ladle over the XO sauce, add the sliced spring onion and toss to combine. Serve hot scattered with extra spring onion.
Note - Shrimp paste, dried prawns and dried scallops are available from specialist Chinese grocers.