1. Melt the chocolate in a heatproof bowl over a small pan of barely simmering water. Once just melted, remove from the heat and set aside until cooled to room temperature.
2. Gradually stir the custard into the chocolate, until smooth. Make sure that the two are the same temperature, otherwise the chocolate may seize up.
3. Whisk the cream until it forms soft peaks. Fold a large tablespoon of the cream into the chocolate mixture to loosen. Very gently fold through the remainder, along with most of the clementine zest, being careful not to knock out too much of the air. Divide between 6 glasses and set in the fridge for at least four hours, or overnight.
4. To serve, sprinkle over the remaining clementine zest and shortbread crumbs.
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