Preheat the oven to 180C, and position a rack in the lower third of the oven.
Line a 15cm round cake pan (with 5cm sides) with parchment paper on the bottom, and lightly grease the sides with oil.
Next, in a medium bowl, whisk together 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 batter into cake pan, and bake for 29-32 minutes, or until a toothpick inserted into the cake comes out clean.
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, 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 on low power..
Pour the chocolate over the cake and make shapes with a spatula.