Place 6 glasses in the freezer.
Peel the skin away from the mango with a sharp knife, then cut all the flesh away from the stone and roughly chop it. Place it in a bowl and blend with a hand-held blender until the mixture is a purée. Add the gin and sloe gin and mix well.
Place in the freezer for 1 hour and then remove and break up the crystals with a fork. Return to the freezer and chill for 10 minutes before again breaking up the crystals with a fork.
Repeat this process at least 5 or 6 times, or until the crystals are firm enough to hold their shape. Fill the chilled glasses with the crush and return it to the fridge for 10 minutes. Serve at once with a small nib of dark chocolate.