Break up the chocolate cake bars and divide between four glass bowls or glasses. Pour over the whisky cream liqueur and leave to stand for 15 minutes.
Whisk the cocoa powder into the custard until completely smooth. Divide this between the four bowls, covering the chocolate cake bars.
Add a big dollop of whipped double cream on top of each one then sprinkle with the grated chocolate. Chill for 15 minutes before serving.
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