« » 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.
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.
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.
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.
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.
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.
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.
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.
For the crumb coat, spread the entire cake with a thin coat of frosting using an offset spatula. Chill for at least 30 minutes.
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.