Preheat the oven to 180°C, fan 160°C, gas mark 4. Thoroughly grease 4 ramekins.
Melt the butter in a large pan. Add the flour and cook gently for 1-2 minutes, stirring continuously. Gradually add the milk, stirring all the time, until you have a smooth sauce.
Remove from the heat and stir in the grated Leerdammer. Cool slightly and add 2 of the egg yolks, beating well to form a smooth, glossy, thick sauce.
In a large clean bowl, whisk the egg whites until stiff, and then gently fold into the mixture using a large metal spoon. Add seasoning to taste.
Spoon the soufflé mixture into the prepared ramekins and bake for 15-20 minutes or until well risen and golden brown.
Serve immediately for a delicious starter or light lunch with a side salad.
Cook's tip: For a perfectly-risen soufflé it's important to heat the oven before baking and try not to open the oven door until the soufflés are almost cooked. They can be left in the oven for a few minutes once cooked as long as the heat is turned down low.