To make icing: In a small food processor or in a bowl and using a whisk or an electric mixer, beat all icing ingredients until smooth. Chill while preparing cakes.
Preheat oven to 350°F Spray two 9-inch round cake pans with cooking spray.
To make cake: In a small microwave-safe bowl, heat chocolate chips and water on High in the microwave for 30 seconds or just until chips begin to melt. Stir until smooth. In a large bowl and using a whisk or an electric mixer, beat together chocolate mixture, sugar, cocoa, egg, egg whites, sour cream, corn syrup, oil, and vanilla. In another bowl, stir together flour and baking powder.With a wooden spoon, stir flour mixture into chocolate mixture just until combined. Divide between prepared pans.
Bake in centre of oven for 15 to 20 minutes or until a tester inserted in centre of cake comes out clean. Cool in pans for 10 minutes on a wire rack. Invert cakes onto a wire rack to cool completely.
Place one cake on a serving platter. Spread some icing over top. Top with remaining cake; spread icing over top and side of cake. Sprinkle with icing sugar or flaked chocolate.