Preheat the oven to 400°. In a saucepan, whisk the mole paste with the stock. Boil the mole sauce over high heat, whisking occasionally, until reduced to 2 cups, 25 minutes. Season with salt and pepper.
In an ovenproof skillet, heat the oil until shimmering. Season the chicken breasts with salt and pepper and add them to the skillet, skin side down. Cook over moderately high heat until browned and crisp, 4 minutes. Turn the chicken and cook for 3 minutes. Transfer the skillet to the oven and roast the chicken for 20 minutes, until just cooked through. Let the chicken rest in the skillet for 5 minutes, then transfer to plates. Spoon the mole sauce alongside, garnish with sesame seeds and serve.
Notes: Black mole paste is available at Latin markets or at mexgrocer.com. Serve With Rice pilaf.