Someone wants a banna cake with choc genache filling. Wouldn't it oooze out?
When it firms up (make it a little thicker than usual) its perfect. I use it all the time as a filling. Doing that this weekend as a matter of fact.
Instead of using it right away like a glaze, wait until it cools completely. Do not cover it though or else the water condensation can seize up the chocolate. Some people prefer to make the ganache the night before it's used to get the right consistency. Also, do a thick ring of buttercream on the outside rim of the cake layer, just like you would do for a fruit filling. HTH