General tips
Make the cheese sauce on the stovetop, then bag with the cooked pasta. Sous vide can hold the temperature, but it can't build the roux. Make a classic béchamel-cheese sauce, fold in the pasta, then bag and bath at 165°F for 1 h to set.
Use a mix of sharp cheddar, gruyère, and parmesan. Single-cheese mac is one-note. The combination gives you sharpness from cheddar, nutty depth from gruyère, and salty umami from parmesan. About 60% cheddar, 30% gruyère, 10% parmesan by weight.
Add a teaspoon of dijon mustard to the sauce. It sounds wrong but it sharpens the cheese flavor without tasting mustardy. Almost every great mac and cheese recipe includes it. Trust the move.
Top with breadcrumbs and broil at the end for the crispy crust. Bagged mac is creamy and silky, but it needs that textural contrast on top. Mix panko with melted butter, sprinkle on top, broil 3 min until golden. Mandatory.
Anti-tip: don't use pre-shredded cheese from a bag. Pre-shredded cheese is coated in cellulose and other anti-caking agents that prevent it from melting smoothly. The result is a grainy, broken sauce. Buy a block and shred it yourself - it takes 90 sec.
Creamy & Smooth tips
Use evaporated milk for ultra-creamy sauce that won't break
Mix sharp cheddar with American cheese for best melt
Don't overcook pasta - it continues to cook in the bag