1. Panko breadcrumbs are larger pieces than normal breadcrumbs which creates a really nice extra crunchy topping. It can be found in the Asian section of supermarkets here in Australia. Can sub with normal breadcrumbs.
2. Cheese
3. Tossing pasta with butter provides a light coating that helps prevent it from bloating while baking in the sauce. This is also the reason why I let the pasta cool before mixing in with the sauce. Optional step – can skip it.
4. Baking dish size – 2.5L/2.5qt (10 cup) baking dish. A 9 x 13″ / 23 x 33 dish is perfect for a double batch, a little too big for a single batch (spreads too thinly, you want more depth).
5. PREPARE AHEAD: Mac and Cheese is best made fresh but if you need to make ahead, this is the best way: Cook pasta and toss in butter, cool (this is important as hot pasta will absorb sauce and bloat). Make cheese sauce per recipe, mix through pasta, sprinkle on breadcrumb topping. Refrigerate or freeze (thaw fully in fridge before baking). Cover with foil and bake 15 minutes at 180°C/350°F. Remove foil then bake further 25 minutes until top is golden. (Fridge cold Mac & Cheese takes longer to bake).
REHEATING LEFTOVERS: Best way is to microwave to make warm then spray top with oil (or butter) and crisp under grill/broiler.
Taken from: https://www.recipetineats.com/baked-mac-and-cheese/