Rinse poussins and pat dry with kitchen paper. Rub the bird inside and outside with salt and pepper. Gently lift up the skin over the breast and spread about 1 teaspoon softened butter inside. Repeat with the remaining birds.
Melt 2 tablespoons butter in a frying pan and cook and stir shallots and 2 sticks chopped celery until soft. Add the chestnuts. Add garlic and cook and stir for a few minutes. Add the pear to the chestnut mixture and stir. Cook for another minute, then add marjoram and season with salt and pepper to taste.
Place the poussins with the breast side up into a roasting tray. Fill them with the chestnut mixture and secure the cavity closed with cocktail sticks or kitchen string.
Add the remaining celery to the roasting tray together with the onion, carrots and parsley. Finally, pour in the chicken stock and white wine.
Roast for 30 minutes in the preheated oven. Melt the remaining butter and brush the poussins after 30 minutes. Cook for another 30 minutes, basting the poussins every 15 minutes, until birds are browned and crisp and juices run clear.
Remove the poussins from the roasting tray and set on a warm plate. Cover with tin foil and set aside.
Strain roasting juices through a sieve into a small saucepan. Bring to the boil and reduce slightly. Add a few tablespoons of cream and thicken with cornflour.
Split each poussin in half and serve each half with gravy.
Poussin, or spring chicken, is the name given to a small, young chicken only four to six weeks in age. Due to the age, the flavours of the bird have not developed fully and therefore benefit from aromatic and rich stuffing. Depending on the size, one bird may be perfect for a single serving.