1 (18.25 ounce) box Swiss chocolate cake mix
1 1/3 cups water
1/2 cup vegetable oil
1/2 cup milk chocolate chips
2 tablespoons water
1 (8 ounce) container Cool Whip, thawed
Batter: Grease Tiara pan, but do not flour. Add eggs, water and oil to cake mix, blending on low until moistened, then on medium for 2 minutes. Place 3 1/2 cups of batter in the pan (you'll have batter left over to bake cupcakes) Bake the cake at 350 degrees F for 21 to 24 minutes or until done. Cool for 5 to 10 minutes, then remove from pan, inverting cake onto a wire rack. Let cake cool completely before placing on a serving platter.
Mousse: Melt the chocolate chips and water over low heat, stirring constantly until smooth, then remove from heat. Remove 2 tablespoons of chocolate mixture and set it aside.
Refrigerate remaining chocolate for 10 minutes, until thickened but still creamy. Fold it into the whipped topping and spread evenly in the cooled cake. Refrigerate for 1/2 hour, then drizzle the set-aside chocolate over the mousse. Refrigerate 2 more hours.