1
Bring 250 g of milk and the vanilla paste to the boil. Mix the remaining milk smoothly with the egg yolks, sugar, cocoa and starch. When the milk comes to the boil, skim it off and pour some of it over the egg yolk mixture, stirring constantly to temper the yolks, then pour the mixture into the saucepan and cook for a few minutes, stirring constantly, to thicken the cream.
2
Remove from the heat and melt the butter and chocolate in the cream. Transfer the custard to a large plastic container and cover with plastic wrap so that the plastic wrap is touching the custard. Cool the custard to room temperature before putting it in the fridge.