In a medium pot over medium heat, melt the butter. Once almost completely melted whisk in the flour and mustard powder. Cook for a minute to get the raw flour taste out and then slowly whisk in half of the milk.
Cook, while constantly whisking for one minute to get any lumps out, then slowly whisk in the remaining milk.
Whisking every 30 seconds or so, cook the bechamel for 7-10 minutes until it is thick and coats the back of a wooden spoon. If your milk starts to boil, lower your heat. You do not want to break the sauce. The best test for thickness, is to run your finger against the back of the spoon, if the line you made stays, the sauce is ready.
Turn off the heat, and whisk in the cheese a little at a time. Whisk in more, once the previous batch is completely melted.
As soon as all the cheese is melted into the bechamel, stir in the macaroni.
Pre-heat your oven to 375 degrees.
In a 9X13 baking dish, pour in your mac and cheese and spread evenly with the back of a wooden spoon.
Next layer the chili on top, smoothing the surface, so it is even throughout.
Finally, cover with crushed Fritos and bake for 15 minutes to set.
Remove from oven and let sit for 5 minutes before serving.