Preheat the oven to 200C/180C Fan/Gas 6. For the potato cakes, bake the potatoes for 50 minutes to 1 hour, or until cooked and tender.
Meanwhile, heat the oil and half the butter in a large ovenproof casserole over a medium heat until hot. Season the poussins all over and add to the casserole. Cook for 3–4 minutes, turning occasionally, or until all sides are lightly browned. Put the casserole in the oven and roast for 25 minutes, or until the poussins are cooked through and the juices run clear with no trace of pink when the thickest part of the leg, between the drumstick and the thigh, is pierced with a skewer.
Peel the langoustines, separating the heads. Take the vein out of the langoustine tails and set the tails to one side. Crush the heads with the side of a heavy knife and set them aside.
Remove poussins and leave to rest. Place the casserole back on a medium heat and add the langoustine heads, shallot, garlic and carrot. Cook for 2 minutes, then pour in the brandy and wine and flambé. (CAUTION: Keep your face and hair away from the flames. Do not leave unattended.). Add the cream, tomato purée and any juices that have run from the poussin. Boil for 2–3 minutes and pass through a sieve. Set aside.
To make the potato cakes, remove the flesh from the baked potato skins, mix with the spring onions and season. Divide the potato mixture in two and shape into little patties. Dust with flour and fry in the butter for 5 minutes on each side.
Meanwhile, sauté the spinach in frying pan with the remaining butter and season. Reheat the langoustine tails in the sauce and season with salt and pepper.
Cut the poussin in half and divide between two serving plates. Serve with the potato cakes and spinach with the langoustine sauce poured over.