To make the soufflé mixture, in a heavy—bottomed saucepan, melt the butter over medium heat. Add the flour and stir for about 2 minutes, or until the mixture is smooth and smells toasty. Slowly add the milk while whisking constantly, then raise the heat to high and continue to whisk for about 5 minutes, or until the sauce boils, thickens, and is smooth. Remove from the heat and stir in the cheese until smooth, then season with the nutmeg and with salt and pepper to taste. Beat in the egg yolks one at a time and set aside.