Melt the butter in a large pot. Add the flour and stir well. Pour the milk slowly, stirring constantly for 10 minutes over low heat.
Add cubed emmenthal and remove from heat when melted.
Add the yolk, and stir well.
Mornay Sauce
40g butter
40g superfine flour (00 flour)
500ml milk
100g emmenthal cheese
1 yolk