Pre-heat the oven to 200°C. Brush 4 individual, ceramic ramekins with half of the softened butter, using a pastry brush, making upwards strokes as you brush. Chill until needed.
Melt the butter in a saucepan over a medium heat, then whisk in the flour until you have a smooth roux. Cook the roux for 1-2 minutes, whisking occasionally. Whisk in the milk in a slow, steady stream until you have a thickened sauce, then whisk in the egg yolks, mustard and most of the grated cheese until smooth.
Simmer the sauce for 5-6 minutes, adjusting the seasoning as you do, then remove from the heat. Whisk the egg whites in a clean mixing bowl with a pinch of salt until soft peaks form, then fold into the cheese sauce.
Spoon into the ramekins and sprinkle the tops with the remaining grated cheeses. Run the tip of your finger around the rim of the ramekins then bake for 12-15 minutes until golden and risen. Remove and serve immediately.