Preheat two ovenproof blini pans or small frying pans in the oven to 200C/400F/Gas 6.
Place the cheese, egg yolks and watercress into a bowl and mix well.
In a separate bowl, whisk the egg whites until stiff peaks form when the whisk is removed.
Fold the egg white mixture into the cheese mixture and season with salt and freshly ground black pepper.
Remove the pans from the oven, divide the butter between the pans and spoon in the soufflé mixture. Return to the oven and bake for ten minutes, or until risen and golden-brown.
To serve, place the pans onto plates.
