Pour milk in a large heavy-bottom saucepan and bring it to a light boil.

In the meantime beat egg yolks and sugar until pale and frothy. Beat in vanilla and vanilla pudding. Add in few tablespoons of hot milk to heat up the eggs, to prevent curdling.

Pour egg mixture into hot milk, and cook, stirring frequently, for about 5 minutes, until thickens. Remove from heat, stick plastic foil to the top of the cooked pudding to prevent making crust. Cool completely.

In a separate bowl, beat the butter until smooth and creamy and mix in cooled pudding until smooth and lump-free. Divide into 4 equal parts.

Preheat oven to 180C (350 F).

In a small saucepan add water, oil, and sugar and cook around 10 minutes until it thickens a bit.

In the meantime, beat the egg whites until stiff peaks form. I recommend using a stand mixer if you have one, or ask somebody for 1 minute of help if using a handheld mixer.

We are about to pour the hot simple syrup into beaten egg whites. Be careful not to burn yourself. Pour the hot syrup in batches, mix well on low speed after each addition. Sift flour, baking powder, and cocoa powder and with a rubber spatula fold everything together. Gently fold in ground hazelnuts.

Now we are going to bake 5 crusts. Divide the batter into 5 equal parts.

Reverse your 33x23 cm (9×13inch) baking pan, so the bottom side is up. Crumple parchment paper with your hands and put under running water. Drain and unfold parchment paper and line the pan bottom (I water my parchment paper so it is pliable and to stick to the pan).

Spread one part of the batter evenly all over the pan bottom, put into 180 C preheated oven, and bake around 10 minutes. Don't let the edges burn. The crust has to be kind of soft when you pull it out of the oven. It will harden as it cools. Don't remove parchment paper yet. Set aside.

Repeat with the remaining batter. You can do this on silicone mat too, but mark 9x13 dimensions, and spread batter within the lines, and put the silicone mat with batter in the oven.

Transfer one cooled crust, parchment paper down, to a big serving dish.

Spread one part of buttercream, and cover with second crust, parchment paper up. Now, remove parchment paper from the second crust (if you have a hard time removing it, damp paper with a little bit of water). -> Note 1!

Repeat with remaining buttercream and crusts.

Frost the last crust with chocolate ganache.

Leave to set overnight, so the crusts can soften.

Cut in rectangles and serve.

Keep in the fridge in an airtight container up to 5-6 days.

Pour the cream into a small saucepan and place it over medium-low heat for a few minutes, until it gets hot. Don't let it boil.

While the cream is heating, chop the chocolate into fine pieces.

Add the chocolate into the cream. Stir gently to distribute the chocolate through the cream and then let it sit for a few minutes to give the chocolate time to soften and melt.

Vigorously whisk the mixture in one direction until smooth and creamy.

Allow cooling for 5 minutes before pouring over the cake.