Bake and cool a single layer 10 x 3-inch round cake. Place cooled cake on foil-covered cake board. Prepare for fondant by lightly icing with buttercream. Cover cake with white rolled fondant; smooth. Thin a small portion of buttercream icing with water; tint violet. Position 6 in. cake circle on cake top; lightly sponge remainder of cake top and sides with thinned icing using crumbled plastic wrap or waxed paper.
For all fondant decorations, lighten yellow, blue, green, orange and pink fondant in multi-pack by combining with white fondant. Roll out fondant 1/8 in. thick.
Using all 3 sizes of square Cut-Outs, cut fondant cake tiers; trim as necessary to create cake. Attach to cake top with damp brush. Shape fondant "frosting" strips; position on cut-out fondant cake with damp brush. Roll a thin fondant piece for candle; attach with damp brush.
Cut fondant using medium square Cut-Out; trim into rectangle shapes. Attach to cake with damp brush. Mold balloons by pressing fondant into melting plate cavities dusted with cornstarch; immediately unmold and attach to cake with damp brush. Cut thin strips for gift ribbon and bow and balloon strings; attach with damp brush.
To make streamers, roll out fondant 1/8 in. thick. Cut into thin strips in lengths from 3 to 5 in. long; wrap loosely around lollipop stick. Let set 15-20 minutes then slide off and attach to cake with buttercream. For confetti, dip tip 12 in cornstarch; cut dots and attach to cake with damp brush. For bottom border, roll 3/4 in. balls of fondant in various colors; attach with buttercream. Pipe tip 4 buttercream message.