Credit where credit is due. I did not invent the tim tam cake; you can find the recipe here.
I did invent a crafty short-cut. No-name brand cake mix, for sale at my local small supermarket for 71 cents a packet. Super easy to prepare.
I also took the blog author's advice and crushed up a pack of tim tams, scattering them over the filling rather than lining the middle with whole tim tams. Much easier to cut up the cake that way!
Be warned. It's disgustingly tasty.