In a food processor puree two thirds of the salmon with the creme fraiche or sour cream, the butter and the zest and transfer the mixture to a bowl. Chop the remaining salmon and stir it into the mixture with the green onions, lemon juice, salt, cayenne and the snipped dill. Using two teaspoons dipped in cold water, form spoonfuls of the rillettes into oval mounds, arranging three mounds on each of four small plates. (The rillettes may be made one day in advance and keep covered and chilled in the refrigerator.) Garnish the rillettes with the dill sprigs and serve with toast points or croutons.