AI am making a vanilla cake 12x12 square and want to fill it with a chocolate ganache (which I've never done) but not sure of the amount if ganache needed to fill and consistency

Hi! Sounds delicious!  I make ganache to frost my cakes under fondant.  I use a 2:1 ratio.  Chocolate:heavy whipping cream.  and it is really good.  It is the consistency of peanutbutter.  Sometimes it's a little looser but you can put it in the refrigerator or freezer for a few minutes.  If i were to use it as a filling i would then use an electric mixer and mix it.. it would make it a fluffier.

I do exactly what mrsg111 does.  Good luck!!

The quantity depends on the thickness and the number of layers of ganache you need in your cake. Here is a good tutorial on ganache (3 part video):

I also like these tutorials.

I also like these tutorials.