Season the cutlets with salt and pepper. Spread the spinach on the cutlets, fold in half and press to close. Heat the skillet with the bacon fat until sizzling. Add the chicken and cook over high heat, turning once, until cooked through, 6 minutes; transfer to a platter and keep warm. Add the vinegar to the skillet; cook for 30 seconds. Off the heat, swirl in the butter until melted. Spoon the sauce over the chicken; serve.