Remove the Cornish game hens from the refrigerator and let sit at room temperature for 15 to 20 minutes.

Pat the hens completely dry inside and out using paper towels.

In a small bowl, combine softened butter, minced garlic, fresh thyme, fresh rosemary, fresh parsley, and lemon zest until well mixed.

Loosen the skin on the hens’ breasts and stuff half of the herb butter under the skin; rub the remaining butter all over the outside of the hens.

Season the hens with kosher salt, black pepper, and garlic powder, including inside the cavities; tie legs together and tuck wing tips under if desired.

Preheat oven to 450°F; pour olive oil into a roasting pan, arrange hens breast-side up, and drizzle juice of 1/2 lemon around them.

Roast at 450°F for 15 minutes to brown the skin.

Reduce oven temperature to 375°F and roast for an additional 25 to 30 minutes until a meat thermometer reads 165°F in the thickest part of the thigh.

Remove hens from oven and let rest on the pan for 5 minutes; transfer to plates and drizzle with pan juices before serving.