Preheat oven to 180C/350F.
Place 65 grams of hazelnuts on a baking sheet and roast in the oven for 10 minutes, or until the skins have started to darken and blister.
Remove hazelnuts from the oven and place in a clean cloth. Rub and roll them around for 1 minute to remove the skins.
Allow hazelnuts to cool for 5 minutes, then chop them in half with a knife to create small pieces.
In a mixing bowl, cream together 140 grams of softened unsalted butter and 90 grams of icing or powdered sugar.
Add 1/3 of the zest of one orange, a pinch of salt, the chopped hazelnuts, and 180 grams of plain or all-purpose flour. Mix together with your hands until a shaggy dough forms.
Add 15-25 grams of beaten egg to the dough until it reaches a slightly tacky or sticky consistency.
Rest the dough in the fridge for a minimum of 1 hour by placing it between two sheets of parchment paper and pressing it into an even circle.
Preheat oven to 160C/320F.
Roll out the dough to a thickness of 5 millimetres or 1/5 of an inch, using a pastry ring or case to guide the shape and size.
Brush the pastry all over with the other half of the beaten egg, loosened with a splash of milk or water, to stop the base going soggy. Bake for 10-15 minutes, or until golden brown and firm to the touch with a little give. Leave to cool while you make the filling.
In a small saucepan, add 400 millilitres of whipping cream (30% fat content) and heat on low. Stir occasionally to prevent it from catching on the bottom of the saucepan. Do not allow it to boil.
In a large heatproof mixing bowl, break up 370 grams of dark chocolate (48-70% cocoa) into smaller pieces.
Once the cream is hot and producing wisps of steam, remove it from heat and pour it over the chocolate pieces. Gently stir until the chocolate is fully melted.
Add 85 grams of cold butter (cubed) to the mixture and continue to stir until it is fully incorporated and the mixture is smooth.
Carefully spoon the mixture around the edges of the cooled pastry to seal any gaps. Place in the fridge for 10 minutes or the freezer for 5 minutes.
Once the mixture is set, pour it into the tart case and smooth the top with a silicon spatula.
Refrigerate for a minimum of 4 hours, or ideally overnight.
The next day, add finishing touches such as grated chocolate, cocoa powder, chopped hazelnuts, or orange zest.
Carefully slide the tart off of the baking tray and remove the ring from around the base.
Use a blowtorch to gently heat the outside of the ring to melt the chocolate, or use a hot knife to free the ring from the tart.