Preheat the oven to 500°F.

"Spatchcock" the hens: Use a heavy knife or poultry shears to cut along either side of the backbone and remove it. Lay the hens out flat on a cutting board, breast side up, and press down in the center with your palm to crack the breastbone and flatten the hens. Pat dry with paper towels and season generously on both sides with salt and pepper.

Put a large, oven-safe skillet or sauté pan over medium-high heat for 2 minutes. Add the hens, breast side down, and cook for about 5 minutes, until nicely browned on the bottom, pressing down on them with a spatula occasionally so that more surface area is browned. Turn the hens over, then transfer the skillet to the oven. Roast until the juices are clear (not pink) when the thigh is pricked with a small knife and you can easily wiggle a drumstick in its socket, 20 to 25 minutes. Carefully remove the hens to a cutting board and let rest for 5 minutes.

While the hens are resting, return the skillet to medium heat. Add the stock, stirring to scrape up any browned bits on the bottom of the skillet. Cook for 1 minute. Drain the cherries, reserving 2 tablespoons of the syrup, and add the cherries and syrup to the skillet. Cook until the sauce has reduced by about half, about 2 minutes. Swirl in the butter until melted, then taste and season with salt and pepper if necessary. Serve the hens with the cherries and sauce spooned over the top.