To make the base sauce melt the butter in a medium-sized saucepan, then stir in the flour and the curry powder, English mustard or ground nutmeg and cayenne pepper. Cook for 1 minute, stirring constantly until the roux has a sandy texture.
Gradually add the milk, stirring until the sauce thickens, then simmer for 2 minutes. Remove from the heat, mix in all but a spoonful of the 2 cheeses until melted, then beat in the egg yolks. Season well with salt and black pepper.
Whisk the egg whites until they form stiff, but not dry, peaks. (Include the extra egg whites if you want a very high soufflé.) Beat 1 tablespoon of egg white into the sauce to loosen it. This makes it easier to fold in the whites without losing any air.
With a large metal spoon, gently fold in the rest of the beaten egg whites, turning the spoon in a figure-of-eight movement. Butter the soufflé dish and coat the sides with dry breadcrumbs to help the soufflé ‘climb ‘ up the side of the dish as it rises during cooking.
Spoon the soufflé into the prepared dish and sprinkle the top with the reserved cheese. Bake in a preheated oven, 190°C, 375°F, Gas Mark 5 for 20-25 minutes, until well risen and golden brown – the soufflé should wobble very slightly when shaken. Serve instantly.