Heat the oven to 200C/400F/Gas 6. Melt the butter in a heavy-based saucepan, then whisk in the flour. Cook, stirring continuously, for about a minute. Slowly whisk in the milk and bring to the boil, whisking all the time to prevent any lumps from forming.
Quickly whisk in the egg yolks and remove from the heat; season with salt and pepper. Cover with a piece of buttered greaseproof paper to prevent a skin from forming.
Whisk the egg whites with a pinch of salt until they form stiff peaks.
Add a third of the egg whites to the yolk mixture and beat with a whisk until evenly mixed, then gently fold in the remaining egg whites. Spoon the mixture into four well-buttered 8cm/3in diameter tartlet moulds and place in the oven for six minutes, or until the tops begin to brown.
Meanwhile, season the cream with a little salt and white pepper, warm it gently and stir in 100g/3½oz of each cheese until melted.
Place a layer of the cream mixture into each of four heatproof serving bowls then turn the soufflés out into the cream, sprinkle the grated cheese over the top, then place them under a pre-heated grill for 2-3 minutes.
Serve immediately.