For cornish hens, I use the following procedure:
- Pat the hens dry, sprinkle with fine sea salt and allow them to sit uncovered in the back of the refrigerator overnight.
- Allow them to come up to room temperature. Rub good European butter all over the skin, and between the breast and skin. European-style butter has more fat and considerably less water content than American style-butter.
- Allow a large cast iron skillet to come up to searing temp, add a tiny bit of canola oil and sear the birds.
- Add any additional seasoning, herbs and a quarter cup of liquid (stock or water) and transfer to a 325 - 350 degree fahrenheit oven to finish cooking.
- Remove the birds, put the pan back on the stove to make a pan sauce if you want.
I usually just serve the hens right from the skillet. The butter along with low moisture in the skin gets a nice crisp on the skin and searing helps it along quite a bit. You can experiment between 325 and 350 degrees to find the spot where you maintain succulent breast meat with the kind of crisp you want.
What's especially critical is allowing the birds to come up to room temperature or you'll never get them completely cooked near the thigh joints in time for the breast meat to come out perfectly cooked.
The key takeaways here are getting as much moisture out of the skin as you can (lightly salting and allowing to sit uncovered chilled overnight), using a quality fat both on the skin and between the skin and the breasts to help add flavor and retain moisture, and searing the birds to give the skin a head start on the color and crunch you want.
Anything else (times / temp) isn't as much an exact science as it is a preference, as long as you get all parts cooked to a safe temp.
Alternate Method: Sous Vide & Deep Fry Or Broil
If you have an immersion circulator, you can spatchcock the bird(s) and cook them 150 degrees Fahrenheit for 4 hours. This is slightly under the usual temp, but the birds have been pasteurized using this method, so it's safe to go a little under.
Remove them, season, dust with rice flour and put them in a 325 degree fryer for a minute at most, JUST until the skin crisps. Season them as soon as you pull them out, and you have a really good skin. The rice flour adds a really neat texture, but it's optional.
Just be careful handling the bird(s) because the skin will be very fragile after cooking. You can also use a broiler to finish it if you don't have or want to use a fryer.