Chocolate Cake from 'cook Yourself Thin'

My mother's birthday is coming up and she's been working really hard to get in shape. I don't want to dicourage her or make her feel guilty so I thought I'd try this cake instead of a usual chocolate cake. It didn't give a specific number of servings so I just estimated, however the recipe stated that each serving is only 315 calories.

• Ready In: 1 hr
• Serves: 10
• Yields: 1 cake

Ingredients

•  nonstick baking spray with flour
• 3⁄4 cup all-purpose flour
• 3⁄4 cup sugar
• 1⁄2 cup cocoa powder
• 1⁄2 teaspoon baking powder
• 1⁄2 teaspoon baking soda
• 1⁄4 teaspoon salt
• 1⁄2 cup warm water
• 1⁄2 cup low-fat buttermilk
• 1  large egg
• 2  tablespoons canola oil
• 1  teaspoon pure vanilla extract
• 2  ounces bittersweet chocolate, chopped
• 1⁄4 cup heavy cream

Directions

1. Preheat the oven to 350 degrees. Coat a 6-inch round cake pan (2 inches deep) with baking spray.
2. Combine the flour, sugar, cocoa, baking powder, baking soda and salt in the bowl of an electric mixer. Beat on low speed until well combined, about 1 minute. With mixer running on medium speed, add warm water and beat until well blended. Add buttermilk, egg, oil and vanilla, and beat on medium for 5 minutes, scraping down sides of bowl occasionally.
3. Pour batter into the prepared pan, and bake until a toothpick inserted in the center comes out clean, about 45 minutes. Cool in the pan on a wire rack for 10 minutes, then unmold and cool completely on the rack.
4. When ready to serve, place the bittersweet chocolate in a small bowl. Heat the cream in a small saucepan set over medium heat until bubbles form around the edge. Immediately pour over chocolate. Let sit for 1 minute, then stir until smooth and well blended. Pour over cooled cake and spread to evenly coat top. (Chocolate sauce will drip down the sides.) Let set, about 10 minutes, and serve.