Preheat oven to 200C and put in a baking sheet to heat up.
Brush 4 ramekins with melted butter and dust with the white breadcrumbs.
Melt the 50g of butter in a pan and stir in the flour, mustard powder and cayenne pepper. Add the milk slowly and mix whilst bringing the mixture the boil. Boil for up to 2 minutes until it is thick and coming away from the edge of the pan.
Take the pan off the heat and stir in your choice of cheese, egg yolks, season well and stir until combined.
In a bowl whisk the egg whites until stiff. Gently fold into the rest of the mixture.
Spoon into the ramekins. Put the ramekins on a baking sheet and bake for 10 - 12 minutes until golden and fluffy.
Serve with chopped fresh parsley and a side salad.