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Make the frosting: Place chocolate in a medium heatproof bowl. In a heavy saucepan over medium heat, bring cream to a boil and immediately remove from heat. Pour over chocolate and let sit, undisturbed, for 3 minutes. Gently whisk until combined and smooth. Whisk in the corn syrup.
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Let cool to room temperature, then transfer ganache to refrigerator and chill, stirring frequently and scraping the sides of the bowl with a spatula, until slightly stiff and cool enough to spread, about 1 hour. If frosting becomes too firm, briefly place bowl over a saucepan of simmering water and stir until desired consistency is reached. Frosting should be smooth and spreadable, but not runny.
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Make the cake: Preheat oven to 350 degrees. Butter two 8-by-2-inch round cake pans; line the bottoms with parchment paper. Butter parchment and dust with flour, tapping out excess; set aside. Into a medium bowl, sift together flours, baking powder, and salt; set aside.
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In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, 3 to 4 minutes, scraping down the sides of the bowl as needed. Beat in eggs, one at a time, and then beat in vanilla. With the mixer on low speed, add the flour mixture in three parts, alternating with the milk and beginning and ending with the flour; beat until combined after each addition.
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Divide the batter between the prepared pans and smooth with an offset spatula. Bake, rotating the pans halfway through, until the cakes are golden brown and a cake tester inserted in the centers comes out clean, 30 to 35 minutes. Transfer pans to a wire rack to cool 20 minutes. Invert cakes onto the rack; peel off the parchment. Re-invert cakes and let them cool completely, top sides up.
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Make the orange curd: Combine yolks, orange zest, orange juice, lemon juice, and sugar in a heavy-bottom saucepan; whisk to combine. Cook over medium-high heat, stirring constantly with a wooden spoon or heatproof spatula (be sure to scrape the sides of the pan), until the mixture is thick, about 8 to 10 minutes. Let mixture come to a vigorous simmer and cook, continually scraping sides of pan, for 2 minutes.
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Remove saucepan from heat. Add salt and butter, one piece at a time, stirring until smooth. If desired, strain through a fine sieve into a medium bowl. Cover with plastic wrap, pressing it directly onto the surface of the curd to prevent a skin forming. Refrigerate until chilled and very firm, at least 2 hours or up to 1 1/2 weeks.
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Assemble cake: Using a serrated knife, trim the tops of the cakes to make level. Split each layer into two, for a total of four. Place bottom layer on a rotating cake stand and with an offset spatula, carefully top with 1/2 cup of orange curd, leaving a 1-inch border. Place the second cake layer on top, spread with another 3/4 cup of the orange curd. Repeat with the third layer; top with the final cake layer. Insert 3 wooden skewers into the top of the cake to secure.
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For the crumb coat, spread the entire cake with a thin coat of frosting using an offset spatula. Chill for at least 30 minutes.
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Spread the entire cake with remaining frosting, swirling to coat in a decorative fashion. Serve immediately, or refrigerate, covered with a cake dome. Let sit at room temperature for about 20 minutes before serving.
