To make the nut layers: Position a rack in the center of the oven and pre-heat to 350º F. Butter a 17 x 11 inch jelly-roll pan; line the bottom and sides with parchment paper. [Cut slashes in the corners of the paper to help them fold neatly.].
In a food processor fitted with the metal blade, process the hazelnuts, almonds, and confectioners’ sugar until the nuts are finely chopped. [Or buy ground nuts.] In a large grease-free bowl, whip the egg whites until soft peaks form. Gradually add the granulated sugar and whip until stiff, shiny peaks form. Fold in the nuts.
Spread the batter evenly in the prepared jelly-roll pan. Bake until golden brown, about 20 minutes. Unmold the cake onto a cutting board, peal off the parchment paper, and cool completely. The, trim the edges to even out, and cut the cake vertically into six 2 ¾ -inch-wide strips.
Buttercream filling: Heat ¼ cup of the milk in a heavy-bottomed saucepan. Add the cornstarch and whisk to dissolve. Whisk in the sugar, then the egg yolks. Add the remaining ¾ cup of milk and whisk over medium heat until very thick. Remove form the heat and transfer to a bowl set in a larger bowl of ice water; stir and cool. Using a hand-held electric mixer, add the butter, one tablespoon at a time, then add the kirsch.
Faux Fondant Icing: Combine all the ingredients in a small sauce pan; stir over low heat until the glaze is barely warm, 92º to 95ºF. Use immediately.
Assembly: Place the best-looking nut layer on a wire rack, smooth side up. Spread this layer with warm apricot jam and let stand for about 15 minutes. Pour warm fondant icing over the jam letting any excess drip over the sides. Pipe four thin line of chocolate about ¾ inch apart along the entire length of the icing. To make a feathered effect, at one-inch intervals, draw a wooden toothpick in straight line perpendicular to the long lines of chocolate. Let stand until the icing and chocolate are firm.
Meanwhile, place 1 nut layer on a cutting board; spread it with about 3 tablespoons of the buttercream. Repeat with the remaining layers, ending with the buttercream. Spread the remaining buttercream around the sides of the cake. Press sliced almonds onto the sides. Top with the iced layer. Refrigerate uncovered for at least one hour prior to slicing.