1. Line two greased and floured 9-in. round baking pans with waxed paper and grease the paper; set aside. In a large bowl, combine the first five ingredients. In another bowl, whisk the mayonnaise, water and vanilla until smooth; stir into dry ingredients until well blended. Transfer to prepared pans.
2. Bake at 180° for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool for 5 minutes before removing from pans to wire racks to cool completely.
3. In a large bowl, beat shortening and butter until light and fluffy. Beat in the icing sugar, milk and vanilla until smooth. Place one cake layer on a serving plate. Spread frosting between layers and over top and sides of cake.
4. Divide the remaining frosting among six small bowls. Tint each with yellow, orange, green, teal, magenta and black food coloring.
5. Cut a hole in the corner of a pastry or plastic bag; insert round tip #3. Fill bag with yellow frosting. Pipe ribbons and bows around tops of Starburst lollies to make gifts; set aside. With black frosting, pipe "Happy Birthday" on top center of cake.
6. Place cones around top edge of cake for clown hats. On sides of cake, pipe clown faces and collars. Using a variety of frosting colours, piping tips and lollies, create clown hair, faces and collars. Decorate clown hats. Place gifts around bottom edge of cake.
* Editor's Note: Use of a coupler ring will allow you to easily change pastry tips for different designs.