Preheat the oven to 110ºC/225ºF/gas ¼. Put half the butter in an ovenproof saucepan and pop it in the oven to slowly melt until it’s separated – this will take about 10 minutes. Strain the yellow clarified butter into a separate bowl and set aside to cool. Discard the remaining milky-coloured butter.
Heat a little olive oil in a frying pan. Slowly fry the shallots and garlic for 10 minutes, until soft and tender, then remove to a plate. Wipe the pan clean with some kitchen roll, turn up the heat then throw in the livers and most of the sage. Cook the livers for a couple of minutes on each side, until lightly coloured but still a little pink in the middle – if you overcook them they will lose their smooth texture and become grainy.
Pour in the brandy. If you're using a gas hob you can flame it until the alcohol cooks off, but watch your hair! Simmer for a minute or so, then take the livers off the heat and tip them into a food processor with the cooked shallots and garlic. Blitz until you have a smooth purée. Add the rest of the softened butter and continue to blitz, then season well and add the mace. Transfer the mixture to a serving bowl.
Sprinkle the remaining sage leaves over the parfait, then spoon over the clarified butter. Leave the parfait to set in the fridge for 1 hour. It will taste beautiful straight away, but it’s even better if the flavours are left to develop for a couple of days. If the butter seal isn’t disturbed it should last as long as two weeks, though it never lasts that long in my house!
When you are ready to serve, slice up the bread and griddle the little toasts. Pile them onto a board next to your parfait with a pile of snipped cress on the side and dig in.