Preheat oven to 220C/200C Fan/Gas 7.Place a baking tray in the oven to heat up.
For the marinade, make small cuts all over the poussin or chicken pieces with a sharp knife to help the marinade penetrate.
Mix all the marinade ingredients together in a deep ovenproof dish. Smear the cut chicken with the marinade, cover with cling film and set aside in the fridge for 10 minutes.
Cook the chicken in the marinade in the oven for 15 minutes. You may need to turn the pieces halfway through cooking to ensure they brown evenly on both sides.
Meanwhile for the naan bread, mix the flour, baking powder and salt together in a large bowl. In a separate bowl, whisk together the milk, sugar and half the beaten egg, then add this to the flour mixture and knead lightly until it comes together to make a soft dough (take care not to overwork the dough). Cover the bowl with a damp cloth and leave to rest for 15 minutes.
For the sauce, place the tomatoes in a pan with 125ml/4fl oz water, the grated ginger, garlic, cardamom, cloves and bay leaf. Simmer for about 10 minutes over medium heat until the tomatoes have broken down.
Pick out the larger spices, then blend the sauce with a hand-held blender until smooth. (Pass it through a sieve to obtain a really smooth purée.) Put the purée in a clean pan, add the chilli powder and simmer for 12-15 minutes. It should slowly begin to thicken.
Cut the cooked chicken into bite-sized pieces (the chicken does not need to be completely cooked at this point as it will continue to cook in the sauce). When the sauce is thick and glossy, add the chicken pieces and sieve in any roasting juices.
Add about 200ml/7fl oz water (depending on how thick you prefer your sauce) and simmer for 3-5 minutes. Slowly whisk in the butter, a couple of pieces at a time, and simmer for 6-8 minutes, until the chicken is cooked through and the sauce turns glossy.
To cook the naan breads, pour the oil over the rested naan dough and turn it a few times so it is evenly coated. Divide the dough into 8 equal pieces and roll out each one on a lightly oiled surface into a circle about 9cm/3½in in diameter. Gently stretch out the circles of dough to form the traditional teardrop shape, or, alternatively, roll them into 10cm/4in circles.
Place the naan breads on the preheated baking tray and bake for 4–5 minutes, or until they are starting to brown on both sides. You might need to turn the bread to make sure it colours on both sides.
To finish the curry, add the finely chopped ginger, chillies and cream and simmer for a minute or so longer, taking care to keep the heat low so the sauce does not split. Stir in the salt, crushed fenugreek leaves and garam masala. Season to taste with salt and pepper, then add the sugar. Serve the curry with the warm naan breads alongside.