Melt the chocolate in three separate bowls set over three saucepans of simmering water, until smooth and glossy. Remove from the heat and set aside to cool a little.
Meanwhile, whip the cream with an electric whisk until soft peaks form. Divide the cream between the three bowls of cooled chocolate and whisk each until combined.
To assemble the mousse layers, divide the dark chocolate mousse between six glasses. Smooth to level the surface, then repeat with a layer of milk chocolate mousse and finally white chocolate mousse. Chill for 2 hours or until set. Top with maltesers, to serve.
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