Simmer until the chicken is cooked through and the sauce is beginning to acquire a glaze. Add the chopped ginger, green chilies and cream and simmer for a minute or two longer, taking care that the sauce does not split. Stir in the salt, crushed fenugreek leaves and garam masala, then check the seasoning and add the sugar. Slowly whisk in the butter, a couple of pieces at a time, allow the butter to melt while stirring to create an emulsion. Remove from the heat and serve with naan bread or pilau rice.