Dark Chocolate-Caramel Cake with Gold-Dusted Chestnuts

Ingredients:
9 ounces high-quality milk chocolate (such as Lindt or Perugina)
3 ounces bittersweet (not unsweetened) or semisweet chocolate, finely chopped
6 tablespoons sugar
2 tablespoons water
1 cinnamon stick
1 cup plus 2 tablespoons heavy whipping cream
1/4 teaspoon salt
1 cup plus 2 tablespoons (21/4 sticks) unsalted butter, room temperature
2 cups cake flour
2 teaspoons baking powder
1 teaspoon salt
1 cup (2 sticks) unsalted butter, room temperature
1 1/4 cups (packed) golden brown sugar, divided
4 large eggs, separated
2 teaspoons vanilla extract
1 cup sweetened chestnut spread** with vanilla
1/4 cup whole milk
1/4 cup brandy
2 tablespoons golden brown sugar
24 jarred peeled whole chestnuts**; 12 coarsely chopped, 12 left whole (about 7.25 ounces)
1 cup heavy whipping cream
1/4 cup sugar
1/4 cup water
8 ounces bittersweet or semisweet chocolate, chopped
Edible gold dust***

Instructions:
Combine milk chocolate and bittersweet chocolate in medium bowl. Stir sugar, 2 tablespoons water, and cinnamon stick in heavy medium saucepan over medium-low heat until sugar dissolves. Increase heat and boil without stirring until syrup turns deep amber, occasionally brushing down sides of pan with wet pastry brush and swirling pan, about 6 minutes (time will vary depending on size of pan). Add cream and salt (mixture will bubble vigorously). Bring caramel to boil, whisking until smooth and caramel bits dissolve, about 1 minute. Discard cinnamon stick. Pour hot caramel over chocolate; stir until chocolate is melted and mixture is smooth. Let stand until completely cool, stirring occasionally, about 1 hour.
Using electric mixer, beat butter in large bowl until fluffy. Beat in chocolate mixture in 4 additions. Cover and refrigerate ganache overnight.
Preheat oven to 350°F. Butter and flour 9x9x2-inch metal baking pan. Line bottom of pan with parchment paper. Sift flour, baking powder, and salt into medium bowl. Using electric mixer, beat butter in large bowl until fluffy. Beat in 1 cup brown sugar, then egg yolks and vanilla extract. Beat in chestnut spread, then milk. Sift dry ingredients over and gently mix together. Using clean dry beaters, beat egg whites in another large bowl until soft peaks form. Add remaining 1/4 cup brown sugar and beat until stiff but not dry. Fold egg whites into batter in 3 additions.
Transfer batter to pan. Bake cake until golden and tester inserted into center comes out clean, about 48 minutes. Cut around cake to loosen. Cool cake completely in pan on rack.
Stir brandy and brown sugar in small bowl until sugar dissolves.
Turn cake out onto work surface. Peel off parchment. Using long serrated knife, cut cake horizontally into 3 equal layers. Place 1 cake layer, cut side up, on 8x8-inch cardboard square. Brush cake layer with half of brandy syrup. Spread with 1 cup ganache. Sprinkle with half of chopped chestnuts. Top with second cake layer, cut side up. Repeat with remaining brandy syrup, 1 cup ganache, and remaining chopped chestnuts. Top with remaining cake layer, cut side up. Spread remaining ganache over top and sides of cake. Place cake rack on sheet of foil; place assembled cake on rack. Chill while preparing glaze.
Bring cream, sugar, and 1/4 cup water to boil in heavy medium saucepan, stirring until sugar dissolves. Add chocolate and whisk until melted and glaze is smooth. Let cool until thick but still pourable, about 4 hours.
Pour glaze atop cake, spreading evenly over sides. Chill until glaze sets.
Brush 12 whole chestnuts with gold dust. Arrange chestnuts across top of cake. (Can be made 1 day ahead. Cover with cake dome and refrigerate. Let cake stand at least 4 hours and up to 8 hours at room temperature.) Serve cake at room temperature.
*Sweetened chestnut spread is flavored with vanilla. It is available at specialty foods stores amd at some well-stocked supermarkets during the holidays. You can also order it from Joie de Vivre (800-648-8854; frenchselections.com).
**Whole chestnuts are sold in jars and in vacuum-packed pouches. Like the sweet chestnut spread, they are available at specialty foods stores and at some supermarkets during the holidays.
***Edible gold dust is available at many baking supply stores. You can order it from L'Epicerie (866-350-7575; lepicerie.com).