In the bowl of a stand mixer fitted with paddle attachment, mix butter with almond meal, sifted icing sugar and grated lemon zest.
Add lightly beaten whole egg and mix to combine.
Add the sifted flour with salt and mix just to combine.
Flatten the dough into a round disc, wrap in cling film and refrigerate for 6/8 hours.
Roll the dough gently until you have an even 3 mm. thickness.
Gently ease the dough into the sides of a 7.5 inches (19 cm) perforated tart ring ring, use your fingertips to press the pastry into the edges and up the wall of the tin.
Using the knife, remove the overhanging pastry.
Prick the base with a fork and refrigerate for 3 hours (or freeze).
Heat fan oven at 320F - 160°C and bake the tart shell for 22 minutes.
Out of the oven, sprinkle with cocoa butter, remove the pastry ring and let completely cool.
Set aside tro assemble.
Heat milk and heavy cream until 180F - 80°C.
In a small bowl, whisk egg yolks with sugar and corn starch until well combined.
Pour milk/heavy cream over yolks mixture, stir to combine, then cook until thick, mixing with a whisk.
Out of the heat, add hazelnuts paste and mix to combine.
Spread pastry cream in a large bowl or shallow pan, so that the custard layer is fairly shallow. Make sure to cover the entire surface with plastic wrap to prevent a skin from forming on the surface too.
Let quickly cool.
Set aside to assemble.
Melt chocolate and add vanilla seeds.
Heat heavy cream with inverted sugar and empty vanilla pod until 180F - 80°C.
Remove the pod and pour the mixture over the chocolate in 3 batches.
Let cool until 95/100 F - 30/35°C before using (see notes).
Soften pastry cream with a rubber spatula.
Pour into a pastry bag and fill up the tart shell to 3 mm. from the edge.
Smooth with a small offset spatula, spread here and there halved hazelnuts.
Pour the ganache into the tart shell up to the edge.
Refrigerate until completely set.
Garnish with white tempered chocolate, ground and halved hazelnuts and vanilla powder.
Let the tart at room temperature 5 minutes before serving.
Vanilla white chocolate ganache
According to the type of chocolate you will use (Callebaut, Valrhona, Cacao Barry or others) it is possible that the final result will be a bit different from mine.
I suggest you to prepare a little sample, to test the chocolate.