Make the cake
Position a rack in the center of the oven and heat the oven to 350°F.
Spray two 10-inch round cake pans (with at least 2-inch sides) with cooking spray and line the bottom of each with parchment.
Sift both cocoa powders into a medium bowl. Whisk in the coffee and then the honey. Let cool completely.
Sift the flour, baking powder, baking soda, and salt into a medium bowl.
In a stand mixer fitted with the paddle attachment, beat the butter on medium speed. Gradually add the oil, beating until combined. Add the brown sugar and granulated sugar. Raise the mixer speed to high and beat until light and fluffy, about 3 minutes. Add the eggs one at a time, beating well on medium-high speed after each addition. Add the sour cream and vanilla, beating just until combined. With the mixer on low, alternately add the flour mixture in three additions and the cocoa mixture in two additions, beginning and ending with the flour (scrape down the sides of the bowl as necessary). Beat just until combined. Divide the batter between the two pans, using an offset spatula to spread it evenly. Tap the pans once or twice on the counter to settle the batter.
Bake, rotating the pans halfway through, until a wooden skewer inserted in the center of each cake comes out with only a few moist crumbs clinging to it, 40 to 50 minutes. Cool the cakes in their pans on a rack for 10 to 15 minutes. Run a knife along the sides of the pans, invert the cakes onto the rack, and remove the pans and the parchment. Let cool completely.
Make the ganache
Put the chocolate in a large heatproof bowl.
Combine the heavy cream and honey in a 2-quart saucepan. Stir with a silicone spatula over medium-high heat until the honey dissolves into the cream, about 30 seconds. Just as the cream comes to a simmer, remove it from the heat and pour it over the chocolate. Let stand about 1 minute; then whisk until smooth. Whisk in the butter, rum (if using), and vanilla until the butter is melted and the mixture is glossy. Cool the ganache at room temperature for at least 8 hours. It will thicken as it cools. (The ganache can sit, covered, at room temperature for up to 24 hours.)
Frost the cake
Set one of the cake layers on a serving plate. Spread about 1 cup ganache over the surface of the cake. Top with the second cake layer and spread a very thin layer of ganache over the top and sides of the cake to seal in any crumbs. Refrigerate for 5 to 10 minutes to allow the crumb coat to set. Spread the remaining ganache evenly over the top and sides of the cake.
Make the chocolate curls
To decorate the cake with chocolate curls, if using, soften the chocolate in the microwave on high power for 30 to 50 seconds. Scrape a vegetable peeler firmly down one side of the chocolate block to form thick curls. Put the curls on a plate and refrigerate until they’re firm enough to handle. Scatter over the top of the frosted cake.
Make Ahead Tips
For the best spreading texture, the ganache should be made 8 to 24 hours ahead. Keep covered at room temperature.
For the best flavor and texture, the cake should be made at least 1 day before serving. To store at room temperature, wrap each completely cooled layer tightly in 2 layers of plastic wrap and store for up to 2 days.
Up to 10 hours before serving, fill and frost the cake. Keep the cake at room temperature, covered with a dome, until ready to serve.
The cake layers can be frozen for up to 2 weeks: Transfer each completely cooled unfrosted layer to a 10-inch cardboard cake round and wrap tightly in 2 layers of plastic wrap, then in foil, and freeze. Up to 15 hours before serving, remove the foil but not the plastic wrap from the cake layers and thaw at room temperature. When completely thawed, unwrap, fill, and frost the cake.
If you plan to travel with this cake, it’s easier to transport if it’s refrigerated, uncovered, until the ganache is firm. To aid transport, assemble the cake on a cardboard cake round and transfer it to a covered cake carrier once it’s firm. When you arrive at your destination, put the cake on a serving plate and allow it to come to room temperature before slicing.
nutrition information (per serving):
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Photo: Scott Phillips