Heat 300ml milk in a pan until just boiling.
Beat egg yolks, sugar, flour, cornflour, and vanilla seeds until smooth.
Whisk half the hot milk into the egg mixture, return all to the pan, and whisk over low heat for 6-8 minutes until thick.
Cover surface with cling film and chill in the fridge until cold and firm.
Melt 400g chocolate and spread thinly over two pieces of 23x25cm parchment paper.
Chill chocolate in the fridge for 30 minutes until set firm.
Cut the chocolate into 72 rectangles measuring 2.5x6cm using a warm sharp knife.
Pipe 21 tiny dots of crème pâtissière onto 48 of the chocolate rectangles.
Stack two piped rectangles and top with a plain chocolate rectangle to create three layers.
Garnish each napoleon with a dot of cream, a raspberry, and a mint sprig.