These look wonderful, but I seem to be totally dumb and not sure I understand what you are saying at the start. I presume you are using the recipe for the cake and it is this that you are crumbling up and then adding the ganache mixed with the cream. Which means a separate batch of ganache for the piping?
Rosie These look amazing
And Mimmy to answere your question ,
First of all that is a legitimate. Question and you are not Dumb.
In the first part of the video she is using all the of the cake she baked from her recipe Then after you crumble it all up in a big bowl you will add just a few tablespoons of the ganache that you made. You want to have enough where all the cake mixture holds together to much and it will just be a mess and u will need to add more cake to even it out. Looks like she just added 2 Tablespoons. The rest is for piping.
Hope this helped you and made sense
Deana n Texas