Preheat the oven to 200°C.
Smear the guinea fowl with softened butter and season with salt and pepper.
Place birds in a roasting tray and roast for 35 to 40 minutes until juices run clear.
Transfer birds to a platter, cover with foil, and let rest while keeping the tray juices.
Heat the roasting tray juices over medium heat while scraping the pan bottom.
Whisk in the flour and cook for 30 seconds to form a smooth paste.
Vigorously whisk in lemon juice and white wine, then boil until thickened and reduced.
Strain sauce through a sieve into a pan and simmer with tarragon and cream.
Carve the guinea fowl and serve topped with the tarragon sauce.