How should I alter a roasted buttermilk chicken recipe for a 5.5-6lb chicken, when it calls for 3-4lb?

Hi! I'm not exactly sure how much buttermilk the recipe calls for, but as long as there is enough brine to cover and coat the chicken you should be just fine; you could always add a bit of extra buttermilk. You shouldn't need to adjust the brine time—overnight will definitely work. (Also, make sure to pat the chicken dry with a paper towel before roasting to get that crispy skin.) As for the cook time, a slightly bigger bird will need a little bit longer in the oven. It looks like the recipe your working with calls to roast the chicken for about 10 minutes per pound at 400°F (still roasting for 20 minutes at 425°F), so you could probably tack on an extra 20 minutes or (give or take) to roasting at 400°F. The safe internal temperature for a whole roast chicken is 165°F, so I'd definitely check the temperature about 40 or 50 minutes into cooking (and before carving!) using a thermometer to make sure you don't over or undercook it; the juices should also run clear. I hope that helps and have a wonderful Thanksgiving!

Hi! Looking into this for you!