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If its just the breast meat, leave it on the counter for thirty minutes prior to cooking. Roast means high heat. I like 450 degrees Fahrenheit. If you have problems with it drying out, try this, instead of roasting, poach in chicken stock for 20 minutes with the liquid at a low simmer, not boiling, remove, dry thoroughly, heat pan on high heat for three minutes, add splash of vegetable oil, season breasts, add to pan, cook for one minute on each side. DONE. This way, the breasts will be insanely juicy, and they will still have excellent browned flavor.
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Yes, allow it to come to room temperature first. But roasting at 450F is a pretty high temperature. Is it a skinless, boneless breast (ick)? Or is it still attached to the bone? Personally, if it's bone in I would start it at 400 and after 10 minutes turn it down to 350. Baste during cooking. As with any roast it's done when it's done and you measure the internal temperature. The only time I use boneless breasts is when I make a roulade---you can't roll up the bones---but I still keep the skin on.
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