Preheat the oven to 150 C / 300 F / 130 FAN / Gas 2.
Place the duck fat in the roasting dish to melt in the oven.
Peel the garlic and press down on the cloves with the flat side of a sharp knife to crush the cloves.
Pat the chicken dry with kitchen paper and use a sharp knife or carving fork to poke holes all over the skin.
Season the chicken with half of the salt and pepper and place in the hot roasting dish, along with the garlic, thyme and bay leaf.
Cover the dish with foil and cook for about 2 hours or until fork tender.
Put the chicken on a plate or board and leave until it is cool enough to handle.
Remove the skin and shred the meat into small pieces. It's easier to do this with your hands or by using 2 forks. Discard the skin and bones.
Put the shredded chicken in a mixing bowl with the rest of the seasoning and mace.
Strip the leaves from the remaining sprig of time and roughly chop a tablespoonful. Add to the chicken.
Mix in about 2 tablespoons of the cooking fat and juices so that the chicken rillettes stay really moist.
Pack into jars or ramekins and refrigerate for 30 minutes.
Gently heat the butter in a saucepan or heat in 10 second full power bursts in the microwave.
Pour over the butter and add a sprig of thyme if you wish.
Let the butter set in the refrigerator before serving.