Preheat oven to 375 degrees F.
In an oven proof skillet, on high heat brown skin side of guinea fowl in olive oil until golden in colour. Remove guinea hens to a plate. Set aside.
Add shallots to the same pan and sauté until soft. Add pancetta and let it brown a little. Add garlic and sauté 2 minutes. Deglaze pan with Marsala wine and let simmer 2 minutes. Add stock. Bring sauce to a boil. Add herbs. Put guinea hens back into sauce skin side up. Bake hens in oven for 45 minutes to an hour. Baste every 15 minutes with the sauce. Guinea hens are done when the temperature reaches 165 degrees F. Remove guinea hens to a platter. Return pan to stove top and reduce sauce to thicken. Whisk in butter. Check seasoning. Pour sauce over guinea hens and serve.