In a large pot over medium heat melt the butter.
Whisk in the flour, salt, and paprika, stirring constantly, for about 1 minute.
Add the milk a little at a time, whisking it in completely before adding more. Once all the milk has been added, whisk for 1-2 minutes until the sauce is thick.
In a small mixing bowl add the egg yolk. Add a spoonful of the hot milk mixture to the egg yolk and immediately whisk together. Add the egg yolk mixture to the pot of the milk mixture and whisk the mixture together. (This prevents the eggs from scrambling)
Turn the heat down to low and begin to add the cheese. Tear the American cheese into pieces and add to the pot.
Add in half of the cheddar cheese and all of the Italian blend cheese. Stir to combine and melt all the cheese.
When the cheese is all melted and smooth add in the cooked noodles. Stir gently until everything is combined.
Place the macaroni and cheese in a 9" x13" casserole dish.
Top the macaroni and cheese with the remaining cheddar cheese.
Turn the broiler to high and place the casserole under the broiler until just browned. Do not take your eyes off of it or it will burn. Once the cheese is browned remove the casserole from the oven.
Serve.