Preheat the oven to 180ºC, fan 160ºC, gas 4. Grease 4 x 250ml ramekins and line the base of each with a circle of baking parchment.
Melt the butter in a pan. Stir in the flour and cook for 1 minute. Take off the heat and slowly stir in the milk. Return to the heat and simmer for 2 minutes. Remove from the heat and stir in the mustard powder, egg yolks, half the parmesan, and the cheddar and chives.
Whisk the egg whites to stiff peaks then fold into the cheese mixture. Spoon into ramekins and place in a roasting tin.
Pour boiling water into the tin, so it comes halfway up the ramekins. Bake for 20-25 minutes. Remove and let cool. Turn out and remove parchment. Turn the oven up to 220ºC, fan 200ºC, gas 7.
Place the soufflés on a baking tray, sprinkle with the remaining parmesan and bake for 10 minutes. Serve with the dressed watercress and prawns.