Put a heatproof bowl on top of a pan of simmering water, making sure the bottom of the bowl is not touching the water. Put the chocolate in the bowl with the cream and heat gently to allow the chocolate to melt.
Add the butter (cut the butter up in quite small pieces because you want it to melt quite quickly). Give it a stir just to make sure all the butter is melted into the chocolate mixture. Keep stirring until all the butter has melted and the mixture is smooth and glossy.
Add the gin, lime zest and juice. Whisk the ingredients together until smooth and slightly thickened. Spoon into a sealable container, put on the lid and chill overnight in the fridge.
Take teaspoonfuls of the mixture and roll them into balls. Put on a plate lined with baking parchment and chill for an hour or so.
Sieve the cocoa into a bowl and roll the truffles in it to cover. These make a brilliant present.