Grease two large ramekins with butter. Put ¼ on the bread cubes into the base of each ramekin, then top with ¼ of the gruyere and parmesan. Top with the remaining bread cubes and gruyere. In a small bowl beat together the egsg, milk and mustard powder, then pour half of the mixture over each ramekin. Set aside in the fridge for a minimum of 2 hours.
When ready to cook preheat the oven to 180. Sprinkle over the remaining parmesan and bake for 30 minutes. Serve right away.