My Kind of Chocolate Birthday Cake

Ingredients:
4 ounces bittersweet or semisweet chocolate, chopped
1/3 cup (packed) dark brown sugar
2 tablespoons unsalted butter
1/2 teaspoon grated orange peel
1/2 teaspoon coarse kosher salt
2 large eggs
2 tablespoons sugar
1 tablespoon all purpose flour
1/2 cup hazelnuts, toasted, husked, chopped
5 ounces bittersweet or semisweet chocolate, chopped
3 ounces high-quality milk chocolate (such as Lindt or Perugina), chopped
1 1/2 cups chilled heavy whipping cream, divided
5 tablespoons unsalted butter, room temperature
1/2 cup heavy whipping cream
1 tablespoon light corn syrup
4 ounces bittersweet or semisweet chocolate
chopped 1 tablespoon unsalted butter, room temperature
Large chocolate shards or curls (from 1- to 2-inch-thick 8-ounce piece bittersweet or semisweet chocolate)
Additional toasted husked hazelnuts
Candied orange peel strips

Instructions:
Preheat oven to 325°F. Line 13x9x1-inch baking sheet with parchment paper; butter parchment. Combine first 4 ingredients in medium metal bowl. Set bowl over saucepan of barely simmering water; stir until chocolate and butter melt. Remove bowl from over water; stir in salt (mixture will be grainy).
Beat eggs and 2 tablespoons sugar in another medium bowl until pale, about 6 minutes; fold into chocolate mixture. Fold in flour, then hazelnuts. Spread batter evenly on baking sheet. Bake until tester inserted into center comes out with a few moist crumbs attached, about 13 minutes (cake will be thin). Transfer pan to rack. Cool cake completely.
Combine chocolates in medium metal bowl. Bring 1 cup cream to simmer in small saucepan; pour cream over chocolates and let stand 1 minute. Stir until melted and smooth. Chill mixture until firm, about 2 hours.
Place bowl with chilled chocolate mixture over saucepan of barely simmering water until mixture is partially melted (do not stir), about 5 minutes. Remove bowl from over water; add butter to bowl. Using electric mixer, beat until filling is thick and glossy, about 3 minutes. Using clean dry beaters, beat 1/2 cup chilled whipping cream in another medium bowl until peaks form. Fold whipped cream into filling.
Cover 14x5-inch cardboard rectangle with foil. Invert cake onto work surface; remove parchment. Cut cake lengthwise in half. Place 1 cake half on foil-covered cardboard. Spread 1/2 cup chocolate filling over. Top with second cake half. Spoon remaining filling into pastry bag fitted with large plain round tip. Pipe filling atop cake in side-by-side lengthwise rows, then smooth top. Chill until filling is firm, about 1 hour.
Bring cream and corn syrup to simmer in small saucepan. Remove from heat. Add chocolate; let stand 5 minutes. Stir until smooth. Add butter; stir until melted. Let stand until barely lukewarm but still pourable, about 20 minutes.
Place cardboard base with cake on rack set over rimmed baking sheet. Spoon glaze over top of cake, allowing glaze to run down sides. Using offset spatula, smooth glaze over sides. Mound chocolate shards or curls over top and sides of cake. Garnish with hazelnuts and candied orange peel. Refrigerate until glaze is set, at least 2 hours. (Can be made 1 day ahead. Cover and keep chilled.)