Place the poussins in individual oven dishes. Mix the softened butter with the chopped sage and season with salt and pepper. Rub the sage butter all over the poussins. Roast on the middle shelf of the oven for 35–40 minutes or until the skin is golden and crisp and the poussins are cooked through.
Meanwhile, make the sauce. Heat the butter in a sauté pan until melted and foaming, then add the shallots and garlic, stir well and cook for 3-4 minutes to soften. Add the Marsala and simmer until it is reduced by half. Pour in the chicken stock and simmer until the liquor is reduced by half again. Add the chopped sage, stir in the cream and simmer until the sauce thickens. Season with salt and pepper to taste.
Remove the poussins from the oven and set aside to rest in a warm place for 5–10 minutes. Meanwhile, heat the olive oil and butter in a small frying pan over a medium heat. When the butter is melted and foaming, add the sage leaves and cook until crispy. Remove from the heat.
Transfer the poussins to warmed shallow serving bowls, spoon the Marsala sauce around them and scatter over the crispy sage leaves. Serve with some cavolo nero or other leafy green vegetable on the side.