Add the butter, raw scallops and shrimp to a food processor with parsley, bread crumbs, garlic powder, and lemon zest, and pulse to chop coarsely. Transfer to a mixing bowl and add mango, mayonnaise and Parmesan. (If necessary, add more mayonnaise to bind patties.) Season, to taste, with salt and pepper. Form into 2-inch patties.
Heat the oil in a saute pan over medium heat and sear both sides of the seafood cakes, until golden brown. Leave each side undisturbed for about 2 minutes to allow the surface of the patty to set up and therefore avoid breakage.
Serve with a dipping sauce made of the creme fraiche and sour cream which has been whisked together.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.