Preheat the oven to 120ºC . Line 2 large baking sheets with parchment paper and trace a 25 cm circle onto each piece of parchment. Finely grate the toasted hazelnuts (a rotary nut grater works best) into a bowl. Sift in 0.75 cup of the confectioners' sugar and toss to mix thoroughly. In a large bowl, beat the egg whites and cream of tartar on low speed until foamy, about 2 minutes. Increase the speed to high and beat until the whites nearly double in volume and stiff peaks form. Gradually beat in the granulated sugar, 1 tablespoon at a time. Beat until the whites are dense and glossy and form stiff peaks. With a large rubber spatula, lightly fold in the nut mixture until no white streaks remain. Use a tiny amount of the nut meringue to anchor the corners of the parchment to the baking sheet. Spoon half of the meringue onto one of the circles and, with a thin metal spatula, spread into an even layer to fill the circle. Form a second circle with the remaining meringue on the other baking sheet. Bake the nut mixture in the upper and lower thirds of the oven, switching positions once, until lightly browned and crisp throughout when tapped, 1 to 1- half hours. The amount of time it takes to dry out will depend on the humidity of the day. Let cool to room temperature. (The recipe can be prepared to this point up to 2 days ahead.) Wrap the meringue layers, with the parchment still attached to the bottom (for stability), in a large sheet of foil to seal. TO ASSEMBLE THE DESSERT: Peel off the parchment paper from the meringue layers. Set one layer on a flat surface and spread evenly with 2 cups of the (room temperature) Hazelnut Praline Buttercream. Place the second layer, flat-side up, on top of the buttercream and press gently. With a metal spatula, spread the remaining buttercream evenly on the top and sides of the cake. To decorate, press the Hazelnut Praline around the sides of the cake and sift the remaining quarter cup confectioners' sugar evenly over the top or use stencils to create a design. Garnish with the whole hazelnuts. Refrigerate uncovered for at least 1 hour, or up to 24 hours to firm up. Serve chilled. HAZELNUT PRALINE BUTTERCREAM: In a small heavy saucepan, combine the sugar with the water. Bring to a boil over moderately high heat, stirring to dissolve the sugar. Boil without stirring until the syrup reaches the soft-ball stage, 120ºC on a candy thermometer, about 6 minutes. Meanwhile, in a bowl, beat the egg yolks and whole egg on high speed until pale, fluffy and quadrupled in volume, about 5 minutes. Gradually beat the boiling syrup into the eggs in a thin steady stream. Beat until the mixture cools to room temperature, 6 to 8 minutes. Gradually beat in the butter, 4 tablespoons at a time, and continue to beat until the buttercream is fluffy. Fold in the Hazelnut Praline. Use at once or cover and refrigerate overnight. Remove from the refrigerator at least 1 hour before using, to return to room temperature. (Makes about 3- two third cups) HAZELNUT PRALINE: Lightly oil a baking sheet and place the nuts in a single-layer in the middle of the tray. In a heavy saucepan, combine the sugar with the water. Bring to a boil over moderately high heat, stirring to dissolve the sugar. Boil without stirring until the sugar turns golden, about 10 minutes. Drizzle the boiling caramel over the nuts and set aside until hard and cool, about 20 minutes. Break the nuts and caramel into chunks by hitting the bottom of the tray. Grind to a powder in a food processor, about 20 seconds. Use at once or store in an airtight container for up to 3 weeks. (Makes about 2- quarter cups)