In a bowl, beat the egg yolks, sugar and cornstarch until the mixture whitens.
In a saucepan, heat the milk and cream until simmering.
Pour half of this mixture over the chopped chocolate, smooth the preparation. Pour the chocolate mixture into the rest of the milk & cream. Over low heat, stirring constantly, allow the cream to thicken. Pour the latter into the jars two-thirds full. Leave to cool and refrigerate.
Prepare the whipped cream by whipping the cream with an electric mixer. She must be firm. In a piping bag fitted with a fluted nozzle, alternate whipped cream and pistachio supreme. Decorate each liegeois.
Serve as fresh.