For the creme patissiere, beat the egg yolks and sugar together, then add the cocoa and flour, whisking well. Warm the milk and cream in a saucepan, then, whisking, pour this onto the eggs and sugar before pouring everything back into the saucepan on the heat and, stirring constantly, bring it to the boil. When the mixture has thickened, take it off the heat and stir in the chopped chocolate and vanilla. Let it cool now, but avoid putting it into the fridge as it will become too solid. You can stop it forming a skin either by covering with buttered baking parchment or greaseproof paper and allow to cool down completely. Meanwhile, cut the rolled puff pastry into 3 large equal sized rectangles, prick all over with a fork and bake for 30 minutes. When cool, spread a little of the chocolate crème patissierre on one piece of pastry and cover with a thin layer of the chocolate cake. Soak the cake with the liqueur. Spread over half of the remaining crème patissierre and spread out. Place on the second piece of pastry, press down and cover with the remaining crème. Spread out and cover with a layer of fresh raspberries, followed by the last piece of pastry. Dust with cocoa, arrange extra raspberries on top and toss over a few toasted pistachios.