Hi I had copied this from the topic "cookie in the cake" I too saw on here last year in October. Here it is and I hope it helps!
"You just spray the cake pan well w/oil and pat the dough in leaving a space all around the edge for expansion, I use the pre packaged cookie mix and 1 env. makes a 10 - 12", or 1/2 envelope for 8 - 9"(and just use the whole egg the recipe calls for"
I put a cookie in between layers all the time. I just bake a cookie slightly smaller than the circumference of the layer in which you are sandwiching. If it is an 8", I form a 7" cookie and it spreads to about 8". I place the dough down on parchment and to make sure it is level, I flatten it with a cake pan. It is about 1/2" thick in dough form. Bake about 15-18 minutes but watch it closely as you don't want brown edges. Assemble the layer by placing the bottom layer, buttercream, cookie, buttercream, top layer. Don't limit yourself to sugar cookies... I use chocolate chip, white macadamia, oatmeal, and on, and on, and on...
Edited to add that I don't bake mine inside the cake pan... I think it makes for tough edges. I bake mine on a parchment lined sheet just as I would regular cookies --just one big one!
Yep that sounds about right, I do this all of the time
Thanks everyone for your help. I made one when the thread first came out but had not had anyone ask for it in some time. Now my niece want it in her birthday cake and I was tearing my house apart trying to figure out what I did with the directions. 
I should have saved myself the frustration and just come here first. 
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