Peel the sweet potatoes and slice them very thinly. Drop the slices into cold water to prevent discolouration. Make a stack of slices a few at a time and cut them into matchsticks. Drop these in a saucepan of cold water. Add salt to taste and bring to the boil. Simmer two minutes and drain. Rinse with cold water and chill.
Combine the mayonnaise, lemon juice, salt and pepper and cayenne to taste.
Thinly slice the cheese then cut into matchsticks the same way you did the potatoes. Cut the prosciutto into thin strips.
Add the potatoes, cheese and prosciutto to the mayonnaise and gently toss. Chill until serving.