Slash each chicken leg and breast 2–3 times with a sharp knife, then put the chicken in a large plastic bag. Finely chop the ginger, chillies, coriander and garlic in a food processor or with a large knife. Mix into the yogurt, then stir in the lemon rind and juice, spices and salt. Spoon the mixture into the plastic bag, seal well and refrigerate for 4 hours (or overnight).
Allow the chicken to come to room temperature for 1 hour, then remove it from the bag (with a thick coating) and put it in a roasting tin. Spoon 4 tablespoons of water into the base of the tin. Drizzle the chicken with butter.
Roast, uncovered, in a preheated oven, 190°C (375°F), Gas Mark 5, for 1 hour 20 minutes, spooning a little more of the yogurt marinade over the chicken once or twice.
Test the chicken (see