Pre-heat the oven to 200°C.
Grease 4 x individual charlotte tins with most of the butter. Cook the potatoes in a large saucepan of boiling, salted water until tender; roughly 20 minutes. Drain and allow to cool a little before mashing with the cheese, tuna, nutmeg and seasoning until smooth.
Add the egg yolks and mash well again to combine. Whisk the egg whites in a separate, clean mixing bowl with a pinch of salt until stiff peaks form. Fold a third of the egg whites into the potato mixture to loosen it, then add the rest and fold in carefully.
Spoon into the prepared tins and bake on a baking sheet for 15-18 minutes until golden brown on top. Melt the remaining butter just before the souffles are ready and brush the tops of them with it just after you remove them from the oven. Serve immediately.