Meanwhile, in another large saucepan, bring the chicken stock to a boil. Add the fava beans and simmer over moderately low heat until they are tender, about 3 minutes. Add the reserved pancetta and the rabbit and cook over moderately high heat, stirring, until the stock is almost completely absorbed, about 2 minutes. Removed from the heat and stir in the pasta, the mascarpone and the remaining 1 tablespoon of butter. Season with salt and pepper and serve at once.