Special equipment: A 12 by 18-inch jelly roll pan, buttered and lined with buttered paper
Heat the oven to 350 degrees F.
Position a rack at the middle level. Cut the chocolate finely and combine it with the water in a heat-proof bowl. Place the bowl over a pan of hot, but not simmering, water and stir occasionally until the chocolate is melted and the mixture is smooth.
In the bowl of an electric mixer fitted with the whip, beat the yolks with half the sugar until very light.
In a separate bowl, beat the egg whites with the salt until they are just beginning to hold a very soft peak, then beat in the remaining sugar in a slow stream, beating faster.
Mix the chocolate mixture into the yolks, then fold in the egg whites. Pour the batter into the prepared pan and smooth the top. Bake the layer for about 15 minutes, until firm to the touch.
Remove the pan from the oven and loosen the cake with a small, sharp knife. Pulling on the paper, slide the layer on the work surface to cool, about 20 minutes.
For the Ganache: Cut chocolate finely and set aside. Bring cream and butter to a boil; remove from heat and add chocolate. Whisk until smooth and refrigerate until thickened.
For the Praline: Combine sugar and lemon juice in a medium pan and stir well to mix. Place over low heat and stir occasionally until sugar is melted and caramelized. Stir in hazelnuts and scrape mixture to a buttered pan. Allow to cool and harden, then break up and grind to a coarse powder in the food processor.
To assemble cake, cut layer into 2 (9-inch) disks and dice scraps. Whip the ganache on the mixer with the whip to lighten it. Place 1 cake layer on a cardboard and spread with 1/3 of the ganache. Sprinkle with half of the praline and the cake scraps. Sprinkle the cake scraps with more pralines. Spread another third of the ganache on the praline and top with the other layer. Use the rest of the ganache to enclose the whole cake and press chocolate shavings against the side of the cake. Dust top of cake with cocoa powder and mark a lattice pattern in the cocoa with a serrated knife.