To prepare béchamel sauce:
- Melt butter over low heat in a saucepan. Add the flour and stir with a wooden spatula. Cook, stirring, for about 5 minutes, or until the mixture becomes bubbly and turns whitish.
- Pour in cold milk, stir and season to taste. Cook over medium heat until thick, stirring from time to time with the wooden spatula.
- Remove from heat and set aside for 10 minutes to cool.
- Stir in the egg yolk and half the cheese.
- Cook the macaroni in salted water according to package instructions. Drain well.
- In a mixing bowl, combine the macaroni and 3/4 of the béchamel sauce. Mix well and pour into a buttered, ovenproof dish.
- Top with remaining béchamel sauce and sprinkle over the remaining cheese.
- Place on the top shelf of an oven preheated to 210ºC. Bake for around 20 minutes, or until the top is golden brown. Serve warm.