Preheat the oven to 190°C. Generously grease eight 250 ml soufflé dishes and place them on a baking tray.
Melt the butter in a saucepan, add the flour and stir to blend for 1 minute. Pour in the milk gradually and bring to the boil, stirring all the time until thickened. Add the grated cheese, chilli powder, thyme and a generous grinding of pepper. Stir and leave to cool for about 10 minutes.
Blend the egg yolks into the cheese sauce, two at a time.
Sprinkle the egg whites with a pinch of salt and whisk until stiff peaks form. Whisk one-quarter of the egg whites into the sauce. Working gently, fold in the remainder of the egg whites, using a large metal spoon.
Evenly divide the mixture among the prepared dishes and bake for 15 minutes until risen and golden. Serve at once, garnished with sprigs of fresh thyme.