Makes about 80 pieces

300ml/1/2pint double cream
50g/2oz glucose
1 vanilla pod
A generous pinch of sea salt
Zest of one lemon
Zest of 1/2 lime
220g/72/3oz dark chocolate (70 per cent cocoa solids we use Colombian single-estate chocolate by Santander)
30ml/11/4fl oz lemon juice
10ml/1/2fl oz lime juice
40g/11/2oz butter, room temperature

Bring the cream, glucose, vanilla, salt and lemon and lime zest to the boil and leave to infuse off the heat for 30 minutes. Start melting the dark chocolate by placing it in a bowl over a pan of hot water. When it is half-melted, strain 325g (111/2oz) of the infused cream through a fine sieve on to the half-melted chocolate. Mix as for a mayonnaise with a whisk from the centre out until you have a fine emulsion. Add the lemon and lime juice. When it is cooled to approximately 40C (105F), add the soft butter and blend in carefully with a spoon or whisk. To finish, pour it into a baking tray lined with parchment and chill until set; this could take up to 40 minutes in the fridge. Remove the paper, cut into cubes and serve. '