In a food processor, turn hazelnuts into meal. Add powdered sugar and blend well. This mixture is called T.P.T (Tant pour tant is one to one sugar & nut meal). Transfer into a pastry bowl. Cream butter and salt and add T.P.T. Add the egg and process until smooth. Scrap down sides of the bowl as it goes. Add half of the flour; mix to combine and add the other half. Wrap up and chill completely. Butter pastry rings generously. Flour work surface and work out half of the chilled pastry to soften – the dough should remain cool and supple. Roll out pastry into a ≈ 0.12''/ 3mm thick sheet. Cut out into desired diameter, chill and repeat with remaining dough. Shape into tart shell – chill. Trim off excess pastry and store in the refrigerator for a couple of hours or best overnight; uncovered.
Lightly toast nuts for about 10 min. Cook sugar and water to 244ºF/118ºC and toss in warm nuts and salt. Stir constantly on medium high heat. Soon, the sugar is going to crystallize coating each nut – continue to stir for about 10 min, until complete caramelization. Transfer nuts into a clean and hot saucepan and continue to stir. (swapping saucepan helps to get rid of unwanted impurities and crystallized sugar left on sides. Turn the heat off, add cocoa butter – transfer onto silicone mat to cool. Process caramelized nuts into paste (do not over process or oil will separate from the fruits). Set aside. Praliné can last months in the refrigerator or in a cool area.
Process almond paste with sugars and add almond meal. Add butter and cream and add eggs and blend to smooth. Transfer mixture to a pastry bowl. Fold in walnuts and set aside. Beat egg whites and sugar to firm peaks. Fold in walnut mixture. Fill up raw tart shells.
Bake at 400ºF/200ºC for about 20 min. Dust tarts with powdered sugar and garnish with walnuts halves. Serve at room temperature. Enjoy!