Remove all the crab flesh from the shells and mix together the white and dark meat in a bowl.
Melt 75g of the butter in a saucepan, then remove from the heat. Stir in the mace, lemon juice and half the zest and allow to infuse for 10 minutes.
Stir the infused butter through the crab. Season with salt and freshly ground black pepper, then spoon into 4 x 100ml ramekins.
Melt the remaining 50g butter, then stir in the cayenne pepper and remaining lemon zest. Pour into the ramekins, swirling to cover the crab if necessary. Chill in the fridge for about an hour, until the butter has set.
Serve with crusty brown bread or toast.