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.
To help your soufflés rise evenly, run your finger around the edge of the batter just before baking – this will loosen the mixture from the side of the dish.