Preheat the oven to 400 degrees. Roast the almonds and hazelnuts separately for about 10 to 15 minutes or until you get the aroma of roasted nuts. Remove them from the oven. While hazelnuts are still hot, , rub their skins off between 2 kitchen towels. Lower the oven to 325 degrees.
In a food processor, grind the nuts together with the sugar until finely chopped. Do not overprocess or they will turn oily. Fold 2 tablespoons of flour into them and set aside.
Generously butter a non-stick jelly roll pan either 13 x 17 inches or 12 by 15 1/2 inches. Line the pan with silcone treated baking paper or parchment.
Beat egg whites with a pinch of salt until stiff and fold beaten egg whites into the ground nut and sugar mixture along with remaining 2 tablespoons of flour. Spread this on prepared baking pan and bake for 30 to 35 minutes or until golden brown, crisp on top but still pliable.
Remove cake from oven and, using pot holders to protect your hands, immediately turn pan upside down onto a clean kitchen towel. Let cake settle for a few minutes and remove parchment paper. If cake breaks, dont' despair; you can "glue" it back together later with icing.
When slightly cool, cut the cake into 4 long vertical strips, each about 3 inches wide and however long your baking pan was.
The Cake Batter
While chocolate is still warm, mix it with 3/4 cup of heavy cream to form a "ganache"; refrigerate just until cool enough to be of spreading consistency. In a mixing bowl combine praline paste with remaining 1/4 cup of heavy cream and set aside.
Whip remaining cup of heavy cream with confectioner's sugar and vanilla.
Assemble cake: Set one strip of cake on a long board or platter. Set strips of waxed paper underneath edges to catch any icing drips
Spread 1/3 of chocolate cream. Top with second layer of cake, spread with plain vanilla whipped cream.
Spread third strip of cake with praline cream, then place it on top of whipped cream layer. Top with 4th strip of cake.
Refrigerate cake to firm it up before final icing
Frost top and sides of cake except for short ends. Decorate top by making vertical lines or cross hatch marks on top of the cake with a knife or tines of a fork.
Cut off a thin slice from each end to even up ends. Protect ends of cake with plastic wrap and store in the refrigerator until serving time. Cut with serrated knife.