Coat a 10-inch cake pan with vegetable cooking spray. Line the bottom of the pan with a 10-inch parchment paper circle. Coat the top of the parchment paper circle with vegetable cooking spray.
Place both chopped chocolates in a bowl and melt over a double boiler. Combine the sugar and water in a 1-quart saucepan and place over medium-high heat. Bring to a boil until all of the sugar is dissolved. Pour the hot syrup into the bowl of melted chocolate. Whisk until the mixture is homogenous. Add the butter and whisk until well mixed.
Using an electric mixer, beat the eggs and sugar until well blended. Using a rubber spatula, fold this mixture into the chocolate mixture. Be careful to mix gently.
Pour the batter into the prepared cake pan. Place a baking sheet with 1-inch sides in the oven. Place the filled cake pan on the baking sheet. Fill the baking sheet with water so the water covers the bottom inch of the cake pan. Bake until the cake surface appears dull and taut when pressed in the middle, about 40 minutes.
Remove the cake pan from the oven and allow to cool on a wire rack. Turn off the oven and let the baking sheet and water cool before you attempt to remove them.
Decoration: If you would like to decorate this cake, you can make chocolate fans or shave a block of chocolate with an apple peeler. To make chocolate fans, stir the vegetable oil into the melted chocolate. Spread a thick layer of the chocolate mixture onto a marble or granite surface (or use the back of a cold baking sheet). Using a triangle, push across the chocolate surface in a curving motion. Place your thumb on one corner of the triangle as you push to catch the chocolate. The result should resemble a chocolate fan. This takes a little practice and not every fan you make will be usable. Have fun with it!