Over here we call it *an explosion* - because the top of the cake seems as tho it has exploded (in the nicest way) and *something* is exploding out the top.
Hmmm - did that maks sense -
Ganache cake - smooth - refridgerate.
Roll out and Cut round disc shape (1/4 inch thickness) from contrasting coloured fondant/medium.
Place in center of cake.
Roll out 2nd coloured Fondant and cover entire cake.
With scaple/small sharp knife and ruler gently cut as many lines from one side of where dic is to the other.
Wait a few minutes - for the edge of the cuts to harden a bit.
Then using tip of scaple/small knife gently lift up centre of each cut and pull nack to edge of disc.
Then sit what ever *explosion* you want in the middle of disc - so as it appears as tho something is exploding out of cake.
If you want to have stars/flowers/hearts/other shapes exploding out of cake - fill a cake pick with some fondant - stick down in middle of ganached cake - place disc over the top of pick - cover cake - cut and fold back edges - then stick wires with fondant shapes down into cake pick.
(same as in my photos...3 tier black and white cake and green floral cake.)
I use 22 gauge wire.
Hope that helps.