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.
Next, pour in a third of the milk then whisk until smooth. Cook out until thick, whisking/stirring regularly. This will take 2-3 minutes over a medium heat.
Once thick, add half of the remaining milk into the sauce then continue cooking until it has thickened again. Make sure to whisk the sauce regularly.
Add the rest of the milk into the sauce, turn the heat down to low then cook for another 4-5 minutes, until thickened up for a final time. The sauce should be thick enough to coat the back of spoon.
Whilst the sauce is cooking over a low heat, prepare the cheese. To do this, unwrap the cheese slices & grate the cheddar.
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 then serve whilst it's still warm!