Thinly slice the Mars Bars. Place in a bowl with the milk and gently heat over a pan of simmering water, stirring occasionally until melted and smooth.
Cool the melted chocolate bars for about 10 mins, stirring occasionally.
Whisk the cream until it just holds its shape, then gently stir a large spoonful into the caramel mixture. Fold in the remaining cream with a large metal spoon.
Lightly whisk the egg white until stiff but not dry. Fold into the caramel mixture. Pour into 4 chunky glasses or freezer-proof container, cover and freeze for 2 hours.
Remove mousse from the freezer 10 mins before serving. Decorate with grated chocolate.
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