Mix the breadcrumbs into the butter and then mix in the Parmesan. (This two-step process will help make sure that the mixture is fairly cool when you mix in the cheese, as you don't want it to melt into chunks.)**** Plop the chicken pieces onto the baking sheet and coat them with the butter/crumbs/cheese mixture. Press the mixture onto the chicken to get a good layer. If you have any extra crumb mixture, just pile it onto the tops of the chicken pieces.
Bake for about 30 minutes (for boneless chicken piece) or 45-50 minutes (for bone-in chicken). Pour any accumulated juices from the pan onto the chicken once you've placed it on a serving platter, or just serve from the baking sheet and scrape any drippings onto each serving.
*Or, if you really must, you can use boneless chicken breasts. (Obviously you're going to get more meat from 4 pounds of boneless chicken than from 4 pounds of bone-in parts, but you'll still have the same amount of real estate to cover with the butter and crumbs/cheese. So adjust your expectations accordingly as to how many people you can serve.)
**The original recipe calls for 2 sticks. Folks, I'm a great fan of butter, but to call for that much for this amount of chicken is just ridiculous. Sara says that you don't want to run out of garlic butter in the middle of things and has you drizzle the leftover butter on top of the chicken after you coat it with breadcrumbs, yet another fiddly step that may end up disturbing your nice crumb coating you've just worked so hard to achieve.
***Please don't use other store-bought breadcrumbs if you can possibly help it, as they seem so stale. You can pulse up a slice or two of bread in your food processor--and if you have a mini one, so much the better, as then you don't have to mess up a whole big appliance--if you don't have panko. Does everyone know what panko is? They're Japanese-style breadcrumbs, very light and crisp. I try to keep an eye out for panko on sale at my regular grocery store or to see if it's available at Costco, and then I stock up and keep it in the freezer. It's just great to keep on hand. I know you're supposed to make any leftover bread into crumbs and freeze that, but I find that I rarely have leftover bread, and homemade breadcrumbs seem to stick together into a ball when I freeze them. And yes, you do have to freeze them! Don't refrigerate bread unless you like mold.
****If you're using the boneless chicken parts and thus baking the dish for the shorter time, you may want to pre-toast the breadcrumbs so that you're sure they'll get brown and crispy by the time the chicken is done. Just put the breadcrumbs into the oven for 10-15 minutes, stirring them around a couple of times, before you coat the chicken with them.