Place scallops and shrimp in a food processor; cover and pulse until chopped. Add the cream, egg yolk, mustard, salt and cayenne; cover and process until pureed. Transfer to a large bowl; fold in crab and chives. Refrigerate for at least 30 minutes.
With floured hands, shape mixture by 2 tablespoonfuls into 1/2-in.-thick patties. In a large skillet over medium-high heat, cook seafood cakes in oil in batches for 3-4 minutes on each side or until golden brown. Serve with seafood sauce if desired.