In a food processor with a steel blade put flour, butter, powdered sugar, vanilla and salt and process until mixture is crumbly.
Add egg yolk and continue to process just until large clumps of dough form. If the dough is not setting, gradually add a little water.
Transfer the dough chunks on a lightly floured work surface and form a disc-shape with your hands.
Wrap in plastic wrap and refrigerate for about an hour.
Roll out the dough on a floured work surface about 3-4 mm thick and line the pie pan. It is important to create a right angle between the sides and the base of the crust, which will ensure the edges remain high when baking.
Using a fork create holes in the crust. Freeze the crust for about an hour.
Preheat oven to 170c degrees.
Blind baking: put a sheet of heat-resistant plastic wrap or baking paper on the frozen crust and fill with dry beans or special weights for baking.
Bake for about 12-14 minutes.
Remove the weights and the plastic wrap and continue to bake the crust for about 10-15 minutes or until it is golden brown.
Cool completely at room temperature.
Chop the chocolate and put in a bowl.
Heat cream in a small saucepan to a simmer.
Pour the hot cream over the chocolate, wait a minute, and whisk until you get a uniform and shiny chocolate ganache.
Pour the ganache over the baked crust and refrigerate for about 1/2 hour to set.
In a medium pot heat cream and milk to simmering point.
In a separate bowl, whisk egg yolks, sugar, flour and vanilla grains.
Add about 1/3 amount of boiling milk over the egg yolk mixture while whisking until mixture is smooth.
Add the remaining milk and beat well.
Return the mixture into a pot and cook over medium heat white constantly whisking until you get a thick pastry cream.
Cool the cream completely in the refrigerator.
Transfer the cold cream into a bowl and whisk.
In a mixer bowl with paddle-attachment whip butter until it softens slightly. Add praline paste and blend until mixture is smooth.
Add the whipped pastry cream and mix just until creamy and smooth.
Transfer the cream into a piping bag fitted with 2cm serrated catheter and drizzle tall mounds on top of chocolate ganache layer.
Chop the chocolate and melt in the microwave or over double-boiler.
Pour the chocolate onto baking paper or chocolate sheet; flatten to a thin layer using a spatula.
Sprinkle the chocolate with hazelnut bits and let it set a few minutes in the refrigerator.
When the chocolate has stabilized but still slightly soft, cut into 16 cm diameter circle and another circle 6-8 cm diameter, thereby creating a disc with a hole.
Cool completely in the freezer until the set.
Extracting the chocolate disc gently from the paper and quickly put on the tart. If necessary, pipe some more cream in the central part.
The tart is at its best on day of preparation, but you can keep it in a closed container in the refrigerator up to 4 days.
Praline paste can be bought in specialty baking stores.