Heat the oven to Gas 6, 200°C, 400°F. Lightly brush with butter the inside of one 15cm (6in) soufflé dish or four individual ones.
Heat the butter in a small pan, mix in the flour well and cook for one minute, stirring all the time. Remove from heat, add the mustard powder and a little milk and blend until smooth. Slowly add all the milk, blending so no lumps form.
Return to a medium heat, stirring, until sauce reaches a rolling boil. Add the chopped ham and Parmesan (reserving a little). Allow to cool, then mix the egg yolks into the sauce well.
Whisk the egg whites until they are stiff and form peaks. Add the sauce to the egg whites and quickly but evenly fold in. Spoon into the prepared soufflé dishes, sprinkle with the reserved Parmesan and cook for 15 minutes for one large soufflé or eight to 10 minutes for individual ones.
To check if the soufflé is cooked: the centre should be soft and it should have a slight wobble.
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