Bake 2 layers of your favorite firm-textured cake recipe (pound cake works well) in 10.25 x 7.4 in. diamond pan; cool completely. Stack cooled cake layers on foil-wrapped cake boards cut to size using buttercream icing or other favorite filling. Prepare cake for fondant with buttercream icing.
Tint white fondant light blue; roll out to slightly more than1/4 in. thick. Cover with plastic wrap. Marble some of the pre-colored fondant. Roll out marbled and other pre-colored fondant to 1/8 in. thick; cut out butterfly shapes. Lightly brush butterflies with damp brush and arrange over blue fondant. Using rolling pin in varying directions, roll butterflies into blue fondant (shapes should distort) until surface is even; carefully move fondant onto cake. Smooth over cake and cut off excess.
Cut additional fondant butterflies; arrange around bottom of cake for border. Tint small amounts of royal icing to match fondant butterflies. Pipe tip 6 elongated bead butterfly body and dot head; add tip 2 outline antennae.