Preheat the oven to 350ºF.

Cook the pasta al dente according package instructions.

Melt butter on low heat in a small pot. When it starts foaming, add flour and cook for one minute until it begins to bubble, stirring constantly.

Remove from heat and slowly add milk while stirring. Stir on low heat until the mixture begins to thicken.

Remove from heat, add the miso and egg yolk and stir.

Let the mixture sit for 5 minutes, then add 1 cup grated cheese and stir until cheese has melted.

Add the cheese mixture to pasta and stir well.

Butter the sides of a baking dish, add the pasta and remaining cheese on top.

Bake in the oven for 35 minutes until the top is golden brown.