Heat a heavy, fireproof casserole dish over medium-high heat. Add butter and oil. When the butter foam has begun to subside, lay in the chicken, breast down. Brown for 2 to 3 minutes, regulating heat so butter is always very hot but not burning. Carefully turn the chicken to another side, using 2 wooden spoons. Be sure not to break the chicken skin. Continue browning and turning the chicken until it is a nice golden color almost all over, particularly on the breast and legs. This will take 10 to 15 minutes. Add more oil if necessary to keep the bottom of the casserole filmed.