Split the vanilla bean pod, using a sharp knife, and scrape the seeds.  Place the vanilla seeds, vanilla pod, and the milk in a saucepan, and bring to a slow simmer. Remove from heat and let it soak the vanilla flavor while preparing the eggs.  

Beat the egg yolks with the sugar, until pale yellow and fluffy.  Add the cocoa powder, and corn starch, and mix well.

Remove the vanilla pod from the milk, and slowly start adding half of the warm milk into the egg yolk mixture, while stirring constantly.

Return the egg mixture to the saucepan with the rest of the milk, and heat over medium heat, stirring all the time, until it thickens.  Remove from the heat, and use while still hot.

Insert a toothpick into each hazelnut, being careful not to break it. 

Place the sugar in a dry pan, and stir until it melts and gets a nice caramel color. Remove from heat, and let cool just a little.  

Dip each hazelnut in the caramel, and pull up a thin thread of caramelized sugar. You can play with it, and make different shapes. Carefully place the hazelnuts on a parchment paper to harden completely. 

Use the pulled sugar hazelnuts as decoration, and add the rest to the remaining caramel. Stir, and pour onto a piece of parchment paper. Once they harden, crush them into small pieces to use later for the cake assembly. 

Chop the chocolate into pieces. Heat the cream, and pour it over the chocolate. Stir until the chocolate has melted. 

Line a 6-inch cake form with parchment paper. Add the rum (if using), or a dash of vanilla extract to the milk, and stir well.

Dip each vanilla biscuit briefly in the milk, and arrange in a single layer at the bottom of the cake form.

Spread some of the pastry cream on top.  Sprinkle a bit of the caramelized hazelnuts.  Repeat layering tea biscuits, pastry cream, and hazelnuts until you get approximately 5-6 layers.  

Pour the chocolate ganache on top of the layered cake and spread in an even layer.

Arrange the pulled sugar hazelnuts on top, and leave in the fridge for a few hours or overnight.