Melt the butter in a saucepan
Stir in the flour to make a roux
Gradually add the milk, stirring constantly to avoid lumps
Cook until the sauce thickens
Remove from heat and allow to cool slightly
Beat the egg yolks and add to the sauce
Fold in the finely chopped mushrooms
Whisk the egg whites until stiff peaks form
Gently fold the egg whites into the mushroom mixture
Pour into a buttered soufflé dish
Bake at 200°C for 25-30 minutes until golden and risen
