Batter: Grease Tiara pan (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 for 21-24 minutes or until done; cool for 5-10 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.
VARIATION: CHOCOLATE AMARETTO TIARA CAKE: Use the same recipe as above, but substitute a dark Dutch fudge cake mix, add 1/2 teaspoon almond extract to the mousse and sprinkle 1/4 cup blanched sliced almonds on top.
VARIATION: FOREST TIARA CAKE: Substitute devil's food cake mix, bake and cool cake as directed above; Add 1/2 teaspoon almond extract to 8 ounces non-dairy whipped topping, spread and chill like the mousse - Top with 1 can cherry pie filling.