Wash the leaves and pat dry with kitchen paper. Beat the egg white lightly, just to break it up. Have a saucer of sugar ready. Brush the mint leaves all over with egg white then dip them into the sugar, coating them on both sides. Lay the leaves on a piece of greaseproof paper on a dry baking sheet and put them into a very cool oven, (120C, 250F, Gas .5) for about 2 hours to dry out, until they are crisp and brittle. Cool, then store in an airtight tin until needed.
