1. Make the chocolate crème patisserie in advance. Whisk the egg yolks with the sugar, cocoa and cornflour until smoothly combined. Heat the milk in a saucepan until it just starts to simmer, then slowly incorporate it into the egg mix, whisking constantly.
2. Scrape the custard mixture back into the saucepan, then stir over a low heat until it just starts to simmer. Take the pan off the heat, tip in the chopped chocolate and butter, then stir until melted and smooth. Scrape the custard into a bowl, press a sheet of clingfilm onto the surface to stop a skin