To make cake layers: You will need two 4-inch square layers, two 6-inch square layers, two 9-inch square layers, and three 16-inch round layers. A half-batch of batter makes one 4-inch and one 6-inch layer; 1 1/2 batches makes two 9-inch layers; 1 batch makes one 16-inch layer.
Heat oven to 350 degrees. Toast hazelnuts for 8 minutes; rub them in a kitchen towel to remove as much skin as possible. When cool, process in food processor until finely ground, about 30 seconds.
Heat oven to 325 degrees. Brush baking pans with butter. Line bottom of pans with parchment, brush with butter again, and dust with flour, shaking out excess.
Combine chocolate, butter, and 1 cup hot water in a double boiler; melt over low heat, about 5 minutes. Transfer chocolate mixture to an electric mixer.
Stir sugar into chocolate mixture. Let cool to room temperature, about 10 minutes. Add cream and vanilla; stir with a rubber spatula.
Sift together flour, cocoa powder, baking soda, baking powder, and salt. Add ground chestnuts, hazelnuts, and dry ingredients to chocolate mixture. Beat on low speed for 30 seconds to combine. Add eggs one at a time, mixing 30 seconds and scraping sides of bowl after each addition.
Pour batter into prepared pans (filling 4-inch and 6-inch pans 1/2 inch full, 9-inch pans 3/4 inch full, and 16-inch pans with all the batter). Bake 4-inch, 6-inch, and 16-inch layers for about 35 minutes, 9-inch layers about 45 minutes; cakes are done when a toothpick inserted in center comes out clean.
Let cool completely on a rack. Invert onto an appropriate-size cardboard round or square, remove parchment, and immediately reinvert onto another round so cakes are right side up. Wrap with plastic wrap; store at room temperature.
To make ganache: You will need three batches for the filling and one for the glaze (the batch for the glaze should be made just before using). Chop chocolate into small pieces. Melt chocolate with cream in a double boiler over medium heat. Transfer to a stainless-steel bowl. Immediately add butter; stir until butter is melted.
Fold in sour cream until completely incorporated. If not using within 2 hours, cover with plastic wrap, pressing it directly on surface, and refrigerate.
To make sugared fruit: Beat 4 egg whites (or more as needed; use powdered whites if concerned about consuming raw eggs) a few times with a fork. Brush apples and pears with whites and roll in sugar; shake off excess. Let dry on parchment-covered sheet pans. Store at room temperature in a non-humid environment for up to 24 hours. Sugar the grapes, chestnuts, and laurel leaves (paint leaves only halfway; leave the stem end plain) just before using.
To assemble cake: Make three batches of ganache; place over a large bowl of ice and water to cool, whisking continuously; do not let it solidify. Transfer to an electric mixer fitted with the whisk attachment. Whip until soft peaks form.
Spread 1/4 cup ganache on one 4-inch cake layer (on cardboard); place other 4-inch layer (without cardboard) on top. Repeat with other sizes, using 1 cup ganache between 6-inch layers, 2 cups between 9-inch layers, and 5 cups between 16-inch layers. Use an icing spatula to apply a thin coating of ganache to each tier. Refrigerate tiers until firm, at least 1 hour or overnight.
Make one batch of ganache for glazing; it should be warm to the touch (125 degrees.on an instant-read thermometer).
Place 16-inch tier on a wire rack over a baking sheet with edges (so excess ganache can be reused). Ladle 16 to 18 ounces ganache on top of tier; spread with an offset spatula, pushing ganache over edges. Pour more ganache over bare spots. Set that tier aside. Repeat process for remaining tiers, using 4 to 6 ounces for 4-inch tier, 8 to 10 ounces for 6-inch tier, and 10 to 12 ounces for 9-inch tier.
Place a masonite board on cake turn table. Smear board with 2 tablespoons ganache; place 16-inch tier on top. Insert a 1/4-inch-diameter wooden dowel into cake; mark where dowel is flush with glaze; cut five pieces to that length. Insert four dowels in a square pattern, and one in center.
Place 9-inch tier, on cardboard, on top, centered over cake. Repeat dowel procedure with 9-inch tier; place 6-inch tier on top. Repeat procedure; place 4-inch tier on top.
Decorate as desired with sugared fruits and leaves.
Raw eggs should not be used in food prepared for pregnant women, babies, young children, the elderly, or anyone whose health is compromised.
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