Place the water and milk into the large stock pot and heat, while stirring, over medium-high heat until just boiling.
Reduce the heat slightly so that the liquid is just simmering.
Add the macaroni (stirring every 30 seconds) and cook according to the packet instructions until al denté.
Remove the pot from the heat (see notes).
Add the butter, grated cheddar and mozzarella and season with salt and pepper.
Stir until melted.
Serve immediately while the pasta is creamy and cheesy.
Notes
RECIPE NOTES & TIPS
Liquid - when you remove the cooked pasta from the stovetop (before adding the cheese), there should be approximately ¼ - ½ cup of liquid remaining in the pot. If there's less, add up to ½ cup of boiling water and stir. Alternatively, if you have too much liquid, remove the excess until you have just ½ cup remaining.
Pasta - use macaroni or elbow pasta (both of which are very small). Larger pasta will not cook in the amount of time listed and would require more liquid.
Stirring the pasta - stir the pasta every 30 seconds to prevent it from sticking to the bottom of the pan.
Vegetables - if you want to add in some veggies, add a handful of frozen peas or sweetcorn kernels 5 minutes before the end of cooking.
Serve immediately - the pasta continues to absorb the liquid as it sits, meaning that it won't stay as smooth and creamy if not served immediately after making.
Storage & freezing - one pot mac and cheese is best eaten immediately, however, you can save your leftovers for up to 3 days in the refrigerator in an airtight container. This recipe is not suitable for freezing.