How to Make It
Preheat the oven to 325°. Lightly butter a 15-by-17-inch jelly-roll pan with 1-inch sides. Line the bottom with parchment or wax paper; butter the paper. Dust the pan with flour, tapping out any excess.
In a medium bowl, sift the flour with the cocoa powder, baking soda and salt. In a large bowl, using an electric mixer, beat the butter at medium speed until creamy. Add the sugar and vanilla and beat until fluffy. Add the eggs, 1 at a time, beating well after each addition. Scrape down the side and bottom of the bowl. At low speed, beat in the dry ingredients in 3 batches, alternating with the buttermilk.
Pour the batter into the prepared pan and smooth the surface. Bake the cake in the middle of the oven for about 35 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool completely in the pan. Refrigerate until chilled, about 2 hours.
Heat the chocolate in a microwave oven until nearly melted. Let stand for 1 minute, then stir until smooth. Whisk in the heavy cream and 2 tablespoons of the hazelnut liqueur. Refrigerate the chocolate filling, stirring occasionally, until slightly firm, about 2 hours.
Run the tip of a paring knife around the side of the cake. Set a large cookie sheet or cutting board on top and flip. Remove the pan and peel the paper off the cake. Using a 6-inch corrugated cardboard round as a template, cut out three 6-inch cake rounds as close together as possible. Using a 4-inch round cardboard template, cut out 2 rounds of cake; using a 2-inch template, cut out 2 rounds from the remaining cake.
Using an electric mixer, beat the chocolate filling until slightly pale and stiff, about 1 minute. Set one 6-inch cake round on a 6 1/2-inch cardboard round and brush lightly with some of the hazelnut liqueur. Spread 1/4 cup of the chocolate filling on the top and cover with another 6-inch cake round. Brush with hazelnut liqueur and spread another 1/4 cup of the chocolate filling on top. Cover with the last 6-inch cake round; set aside.
Set one 4-inch cake round on the 4-inch cardboard template and brush with hazelnut liqueur. Spread 3 tablespoons of the chocolate filling on the top and cover with the second 4-inch cake round; set aside. Set one 2-inch cake round on the 2-inch template and brush with hazelnut liqueur. Spread 2 tablespoons of the chocolate filling on top and cover with the second 2-inch cake round. Refrigerate all 3 layer cakes until thoroughly chilled, at least 2 hours.
Fill a pastry bag fitted with a 1/2-inch star tip with 1 cup of the Vanilla Buttercream. Pipe a small dollop of buttercream into the center of a lazy Susan and anchor the 6-inch layer cake on it. Pipe the buttercream around the cake to fill any gaps between the layers. Using a large offset spatula, spread a very thin layer of buttercream over the entire cake. Carefully lift the cake off the lazy Susan and refrigerate to set the frosting, about 10 minutes. Fill in between the layers and frost the 4-inch and 2-inch layer cakes.
Return the 6-inch cake to the lazy Susan and frost with 1 cup of the buttercream. Run a large offset spatula under hot water, wipe it dry and use it to spread the frosting on the side of the cake in a smooth, even layer. Insert a plastic straw in the center of the cake and trim the straw flush with the cake. Refrigerate until chilled, at least 2 hours. Frost the 4-inch cake with buttercream. Insert a plastic straw in the center, leaving it protruding. Refrigerate until firm. Frost the 2-inch layer cake with buttercream and refrigerate until chilled.
Transfer the 6-inch cake to a cake plate. Center the 4-inch cake on top, using the straw to help maneuver the cake. Trim the straw flush with the cake. Center the 2-inch cake on top. Fill the pastry bag with the remaining buttercream and pipe a decorative border around the base of each tier, covering the cardboard. Refrigerate the cake until firm. Let the cake stand at room temperature for at least 1 hour before serving.