Generously grease the insides of 8 standard 8cm ramekins (or four 10cm soufflé dishes) with the softened butter. Put about 50g grated Gruyère into one dish, rotate it to coat the inside, then tip the excess into another ramekin. Repeat to coat them all.
Preheat the oven to 200C/Gas 6. Beat the egg whites with a pinch of salt until soft peaks form. Immediately mix one-third of the egg whites into the warm soufflé mixture with a whisk, then, using a large spoon, fold in the rest with one hand while showering in the grated Gruyère with the other. Stop as soon as the mixture is amalgamated.
Spoon the mixture into the ramekins to come 5mm above the rim. Smooth the surface with a palette knife, then use a knife tip to ease the mixture away from the side of each ramekins to help it rise.
Stand the ramekins in a deep ovenproof dish lined with a sheet of greaseproof paper and pour in enough almost-boiling water to come halfway up the sides. Bake the soufflés for 4 minutes (or 6 minutes for the 10cm dishes). Put the cooked soufflés on individual plates and serve at once.