Score the legs of the poussins and remove the wishbones. Make the marinade by combining all the ingredients in a bowl, add the poussin, cover and chill. Leave to marinate for at least 4 hrs.
To make the basting butter, mix the garlic with the softened butter. Add the lemon juice, soft dark brown sugar and achuete oil for colour. Mix well.
Heat the oven to 190C/170C fan/gas 5 and heat a griddle pan over a medium high heat. Remove the chicken from the marinade and pat dry. Season and coat with the achuete oil. Put upside-down on the griddle for 5 mins until the skin is crisp and golden, then flip over and transfer to the oven for 20-30 mins until cooked, checking and basting occasionally with butter. Leave to rest for 5-10 mins before serving.
While the poussins are grilling, make the sinamak by mixing all the ingredients together in a bowl. Set aside.
Cut the banana leaf into two 25 x 12cm rectangles using scissors. Run them under water, then toast in a dry pan, lightly blow torch or grill until toasted (this releases the natural oils and makes the leaf shiny). To serve, put the toasted banana leaf on a plate, top with a poussin, a spoonful of the atchara, a sprinkling of spring onions and serve the sauces and sinamak alongside.