Warm butter, salt and milk until butter has melted. Put flour into a basin and make a well in flour. Break eggs into well and work in some flour. Add warm milk mixture and whisk until smooth.
Refrigerate for 2 hours before cooking. The consistency should be like thin cream. If not, thin with more milk.
Heat frying pan (preferably one designed for cooking pancakes). Wipe pan with a piece of buttered paper for the first pancake. Lift pan from heat, then ladle in batter and swirl to spread it to edges of pan. Set pan back over heat. After 1 minute, lift thin outer edge of pancake with a fine spatula and flip pancake using your fingers or spatula.
When cooked, transfer to a plate in a low oven to keep warm. Repeat process.
Once crepes are cooked, lightly butter ramekins. Layer crepes, Prosciutto, ham, Gruyere and Parmesan in baking dishes.
Bake in a 180 deg C oven until bubbling and top is golden.
