Preheat the oven to 350°F (175°C), and position a rack in the lower third of the oven. Line a 6-inch round cake pan with parchment paper on the bottom, and lightly grease the sides with oil.

In a medium bowl, whisk together the flour, cocoa powder, and baking soda. Set aside.

In a small bowl, whisk together the oil, sugar, sour cream, egg, vanilla, and coffee. Combine the two bowls, mixing just until combined.

Pour the batter into the cake pan, and bake for 29-32 minutes, or until a toothpick inserted into the cake comes out clean. If you under-bake the cake, it will slightly sink, so check it! It will also start to pull away from the sides when it's done.

Remove the cake from the pan by carefully tilting it into one hand, peel off the parchment from the bottom, and then place it on a small cake stand.

In a double boiler, or bowl fitted over a pan of simmering water, combine the chopped chocolate, cream, and corn syrup (if using). Stir over medium heat until melted and smooth.

Alternatively, you can melt the chocolate and cream in the microwave in 25-second pulses on LOW power. Stir between each pulse.

Pour the chocolate over the cake, and garnish with pomegranate arils.