Pre-heat the oven to 180 degrees Celsius
In a saucepan melt the butter, add flour and stir until bubbling. Stir in the milk and cook until smooth.
Stir in the cheeses and heat until melted.
Remove from the heat and beat in the egg yolks one at a time. Season.
Beat the egg whites with the cream of tartar. Do not over beat.
Fold into cheese mixture.
Divide among 4 ramekins.
Place ramekins in a shallow roasting dish. Pour in enough water to come 1 cm from top of ramekin.
Bake for 30 - 40 minutes. Or until well risen and puffy.