Add the butter into a large saucepan then set over a medium heat & leave to melt, stirring frequently. Once the butter has melted, stir in the flour then cook out for a minute, stirring constantly.
Gradually add the milk in, a third at a time, letting each addition of milk thicken up before adding in more. Make sure to whisk regularly. Once the last addition of milk has been added, turn the heat down to low then cook out gently until thick. This will take 4-5 minutes.
Once the sauce is thick, stir in the jalapeño juice, mustard, garlic powder & cayenne. Season to taste with salt & pepper now as well.
To finish, take the pan off the heat, add in the cheese then stir to melt. Give the sauce a quick blend with a stick blender, to make it as smooth as possible.
Transfer a quarter of the sauce to a smaller pan then set both pans to one side whilst your pasta finishes cooking.The smaller amount of cheese sauce is for the nachos! Whilst the cheese sauce is cooking, set a large pan of salted water over a high heat and bring to the boil. Once the water's boiling, add in the macaroni then cook until just tender (al dente). This will take around 8-10 minutes.Make sure to stir the pasta once you've added it into the water, to prevent it sticking together. I'd recommend getting the pan of water on for the pasta once you've added the last addition of milk to the sauce. Once the pasta is cooked, drain then add to the larger pan of sauce. Stir to combine then transfer to a lightly buttered 9x13" baking dish.