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Melt the chocolate on low to medium heat in the microwave, checking after 2 minutes, though it will probably need 4 minutes. Or you can place it in a bowl over a pan of simmering water. Once the chocolate is melted, ser aside while you get on with the custard.
In a saucepan warm the milk and cream. Whisk the egg yolks, sugar, and cocoa in a large bowl. Pour the warm milk and cream into the bowl whisking it into the yolks and sugar mixture. Stir in the melted chocolate, scraping the sides well with a rubber spatula to get all of it in, and pour the custard back into the rinsed saucepan. Cook over a medium heat until the custard thickens, stirring all the time. Make sure it doesn't boil, as it will split and curdle. Keep a sink full of cold water so that if you get scared you can plunge the bottom of the custard pan into the cold water and
whisk like mad, which will avert possible crisis.
The custard will get darker as it cooks and the flecks of chocolate will melt once the custard has thickened. And you do need this thick, so don't panic so much that you stop cooking while it is still runny. Admittedly, it continues to thicken as it cools and also when it's chilling in the refrigerator. Once it is ready, pour into a bowl to cool and cover the top of the custard with cling wrap to prevent a skin from forming.
When the custard is cold, pour and spread it over the chocolate cake layer in the trifle bowl, and leave in the refrigerator to set, covered in plastic wrap overnight.
When you are ready to decorate, softly whip the cream for the topping and spread it gently over the layer of custard. Grate the chocolate over the top.