Preheat the oven to 425°F. Lightly grease six 4-inch tart pans (with removable bottoms) with cooking spray and place on a large baking sheet, set aside.

To make the crust, place the flour, powdered sugar and salt in a food processor; pulse a few times to evenly combine. Add the butter and process until the dough comes together in clumps. Divide evenly among the prepared tart pans and then press the dough evenly on the bottom and up the sides of each pan. Freeze for 15 minutes. This will prevent the crusts from shrinking. Prick holes on the bottom of each tart and then bake until lightly golden brown around the edges, about 15 to 18 minutes. Remove from the oven and immediately brush each tart with the egg wash. Set aside to cool completely.

To make the filling, combine the cream, hazelnut liqueur (if using) and the vanilla extract in a medium saucepan. Set over moderate heat and bring to a slight simmer. In a large bowl, combine the chocolate, butter and espresso powder. Pour the hot cream into the bowl, over the chocolate and allow to sit for 5 minutes before whisking until smooth and glossy.

Carefully spoon the chocolate filling to the cooled crusts, filling up to the top of each. Sprinkle each tart with the chopped hazelnuts and a pinch of salt. Chill in the fridge for at least 1 hour and a half before serving.