Heat oven to 350°F (325°F for dark or nonstick pans). Spray bottoms and sides of two 8-inch round cake pans with baking spray with flour. In large bowl, beat cake mix, water, oil and eggs with electric mixer on low speed until moistened, then on medium speed 2 minutes, scraping bowl occasionally. Pour into pans. Bake 14 to 20 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes. Remove from pans to cooling racks. Cool completely, about 1 hour.
In large bowl, beat filling ingredients with electric mixer on high speed until light and fluffy. On serving plate, place 1 cake layer, rounded side down. Spread filling over cake, spreading slightly over edge. Top with second layer, rounded side up.
In 1-quart saucepan, heat whipping cream over low heat until hot but not boiling. Remove from heat; stir in chocolate until melted. Cool about 10 minutes. Pour ganache onto top of cake; spread with spatula so it just begins to flow over edge of cake. Store loosely covered.