Make the ganache: Heat the heavy cream, chocolate and salt in a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water), stirring, until the chocolate is melted and the ganache is smooth. Remove the bowl from the pan and stir in the marmalade. Refrigerate until thick but not set, 1 hour to 1 hour, 30 minutes.
Assemble the cake: Place 1 cake layer on a platter; spread with 1 tablespoon marmalade. Beat the chilled ganache with a mixer on medium-low speed until just fluffy, about 1 minute (do not overbeat). Spread 1 heaping cup of the whipped ganache over the marmalade. Top with the second cake layer. Cover the whole cake with the remaining whipped ganache and refrigerate until ready to serve.
Make the caramel: Bring the sugar and corn syrup to a simmer in a medium saucepan over medium-high heat. Cook, swirling the pan but not stirring, until the mixture turns amber, about 4 minutes. Remove from the heat and carefully whisk in the heavy cream and butter until smooth. Set aside, stirring occasionally, until cool but still pourable, 15 to 20 minutes. Drizzle over the cake and sprinkle with sea salt. Serve the cake within 2 hours; slice with a warm knife.
Preheat the oven to 350 degrees F. Coat two 9-inch-round cake pans with cooking spray and line the bottoms with parchment paper.
Whisk the cocoa powder and 1 1/2 cups boiling water in a medium bowl until smooth; set aside. Whisk the flour, sugar, baking powder, baking soda and salt in a large bowl until combined. Add the eggs, vegetable oil, sour cream and vanilla and beat with a mixer on medium speed until smooth, about 1 minute. Reduce the mixer speed to low; beat in the cocoa mixture in a steady stream until just combined, then finish mixing with a rubber spatula. (The batter will be thin.)
Divide the batter between the prepared pans and tap the pans against the counter to help the batter settle. Bake until a toothpick inserted into the middle comes out clean, 30 to 40 minutes. Transfer to racks and let cool 10 minutes, then run a knife around the edge of the pans and turn the cakes out onto the racks to cool completely. Remove the parchment. Trim the tops of the cakes with a long serrated knife to make them level, if desired.
Courtesy of Food Network Magazine