This is the original recipe for the icing I don't see how it would be enough to coat 12 pieces of cake.
My recommendation is to double it for ease of coating.
Doubling the icing mix also allows you to coat about 16 to 20 squares of cake with a little left over.
I used a butter cake packet mix. You can use a made or bought sponge.
Or see my butter cake recipe here to which you will definitely need to double the icing mix.
This freezes well.