Wash the snails in their shells in cold water. Add periwinkles to a pot of boiling water in a saucepan, along with a small handful of salt to shrink and toughen the meat. (This eases their removal.) The snails are ready when the operculum falls off.
Remove the periwinkles from their shells with a nutpick or pin.
Beat the egg and mix with flour, snails, and salt, until flour is evenly dampened. Form the batter into little cakes with wet hands.
Fry cakes in a well-greased frying pan until golden brown on both sides. Serve with crusty bread and a salad of wild seaside plants, preferably invasive.