Tart Shell:
1. Place butter, sugar and salt in a mixer and paddle until smooth. You can also do this with your hands, mix and blend until smooth.
2. Add the egg and incorporate.
3. Add the flour and mix until smooth. You can split the dough into four equal parts and place in the fridge to cool and make it easier to roll out. About 30 minutes. But can be over night or you can make days ahead and keep it frozen.
4. Remove from fridge and start to roll it out on a floured counter. Roll the dough out enough to cover your tart shell.
5. Bake the shells at 325F (162C) for 15 minutes, they should be light brown, not white, not dark.
6. Remove from the oven and let cool. You can place in fridge to help cool them.

Filling:

1. Chop chocolate into small pieces and place in a bowl with the hazelnut paste.
2. Boil heavy cream then pour over the chocolate and hazelnut paste. Let sit for 30 seconds. Whisk and if the chocolate is not melted, place the bowl over hot water (double boiler) and let the steam heat the cream, chocolate, hazelnut mixture.
3. Whisk until creamy then pour over already baked and cooled tart shell.
4. Place in the refrigerator until it sets, about 2-4 hours but can be overnight.
5. Nice to serve with whipped cream and chocolate shavings.