Melt butter and add leeks.
Cook about 8 mins.
In a bowl make the custard by mixing cream and eggs.
Stir in cooled off leeks, cheese, salt/pepper, prosciutto.
Divide this into your parchment lined cupcake tins or tart pans.
Bake at 400 for approx 15-20 mins for muffin size. Times will vary depending on the size pan you are using so keep an eye!