In a large pot, add the butter and flour. Stir until this turns into light brown color and slightly thick just like a roux.
To this, add the milk. Keep stirring for about 5 minutes until the mixture starts to thicken.
Once this mixture starts boiling, start adding in the cheeses. Add a total of 4 (2 cheddar and 2 gruyere mixed).
Add the cheeses 1 cup at a time. Leave the remaining 2 cups layer into the pasta before baking. Stir until creamy in consistency. Add the mustard, paprika, garlic powder, salt and pepper.
To this, add the cooked pasta and mix it all together.
In a large baking pan (roughly 9x13) which has been buttered, add half the pasta topped with 1 cup of cheese then the remaining pasta with the rest of the cheese on top followed by the panko.
Bake in a 350 F oven for about 25 minutes then broil for 1a minute until the top is lightly browned. Enjoy warm!