Refreeze 1 pastry half, as it’s not required. Place the 3 remaining halves on a lined baking tray, prick all over with a fork then refrigerate for 1 hour.
To make creme, place egg yolks, flour and 2 tablespoons of the sugar in a bowl and whisk until pale. Combine milk, remaining sugar and split vanilla bean in a saucepan and bring to the boil over medium heat. Strain into egg mixture and discard vanilla bean. Return to pan and cook over low heat, stirring for 2-3 minutes or until thickened, then cool.
Preheat oven to 190°C. Bake pastry for 10 minutes, then place a sheet of baking paper and a baking tray on top of pastry and return to oven for 10 minutes. Remove paper and tray and cook for a further 5 minutes, then allow to cool.
Place dark chocolate in a heatproof bowl over a saucepan of simmering water (don’t let the bowl touch the water). Once the chocolate has melted, stir until smooth. Cool completely.
Spread chocolate over one pastry half. Combine whipped cream and creme patisserie. Place an unglazed pastry half on a serving tray and spread with half the cream. Top with other piece of unglazed pastry and remaining filling. Top with glazed pastry. Place white chocolate in a piping bag and pipe thin lines across the top. Use a skewer to drag the lines. Refrigerate for 30 minutes before serving.