Melt the butter in a small frying pan over low heat. Add the flour and cook for 1-2 minutes or until sandy like in texture. Gradually whisk in the milk, add the 1/4 cup of cheese and cook, stirring, for 3-4 minutes or until thickened and set aside.
Spread half the bread slices with the mustard and top with the extra cheese and then ham.
Spread the remaining slices with the pre made cheese mixture and sandwich both sides together. Spread the sandwiches with butter on both sides.
Heat a large non-stick frying pan over high heat and using extra butter, cook sandwiches (pending on pan size in batches ) for approx 2-3 minutes each side or until golden and the cheese is melted.