Preheat oven to 180 celsius. Grease 8 inch tart tin. Set aside. In a food processor, add flour, hazelnut meal, powered sugar, salt, pulse until combined. Then slowly add the butter, pulsing all the while, until you get a coarse meal. Then slowly add the cold milk and pulse until a dough ball forms. Flatten dough into disks, roll dough into a large circle, then place gently into tart tin. Bake for 15 minutes until crust is golden brown. Transfer to a wire rack and let it cool.
In a separate medium bowl, mix milk, cornstarch and melted chocolate until combined. Pour chocolate-milk mixture to the warm hazelnut mixture, mix until it is well mixed. Place mixture back into the saucepan and reheat until thicken, about 1-2 minutes.
Pour pastry cream into the cooled tart. Place in the fridge for at least 2 hours to set.
Make the chocolate ganache, Stir the melted chocolate and coconut cream, folding them together. Keep stirring until combined. Cover with plastic wrap and refrigerate overnight. (Ganache should be thick and spoonable)
Combine butter and chocolate ganache in the stand mixer bowl with the paddle attachment. Whip on medium to high speed for about 3 minutes. Stop mixer and add the sifted powdered sugar. Mix on low speed until incorporated , scrape the bottom and sides of the bowl and then mix on high speed for 3 minutes until smooth
Pipe whipped chocolate ganache over the pastry cream, garnish and enjoy!