Preserving meat by encasing it in fat is an age-old technique found across the globe. Potting is the traditional British version. Originally, meats were potted by being cooked in large jars, sealed with suet, then strewn with herbs to deter insects. The modern method is simpler and done on a smaller scale. You can pot any meat or fish, and it's a great way to use up leftovers. Modern potted meats are not really preserves, but will keep in the fridge for several days, so are good to make in advance.
Begin by clarifying the butter. Butter is only about 80 per cent fat and clarifying it involves melting it so the milk solids and water in it can be removed. The clarified fat, which lasts longer than unclarified, is used to bind and seal the potted meat, preventing spoilage by sealing out all air. Using clarified butter is not strictly necessary for a potted meat that will only be kept a few days, but it is traditional.
Melt the butter over a gentle heat in a small pan and simmer for about 3 minutes, taking care it doesn't brown. This drives off the water in the butter. A foam will form on the surface and a milky liquid will collect at the bottom, leaving clear, golden fat in between. Skim off the foam with a spoon, then pour out the clear butter into a jug, and discard the milky residue. Leave to solidify.
Roughly chop the chicken and ham and place in a food processor. Starting with cooked meat means any bacteria should have been eliminated. Process till fine. Add 100ml of the clarified butter, the spices and salt and pepper, and process to combine. Reducing the meat to a paste with the butter means it can be packed into pots without the air holes that could lead to spoiling. The spices were originally another way to preserve the meat, but are now just for flavouring. Pack the mix tightly into 6 ramekins or ceramic pots of 100-150ml capacity. Remelt the leftover clarified butter. Put a bay leaf in each pot and pour the butter over the meat to seal it fully. Cool, then chill for up to a week (without breaking the seal). Serve one pot per person, with melba toast, as a starter, along with a parsley and fennel salad.