First, pre-heat your oven to 325°F/165°C. Then, lightly grease a 12-inch round deep dish pan with oil or cooking spray. THIS is the deep dish pan I use and it's perfect for this cookie cake. Otherwise, I imagine a 12-inch cast iron skillet would work, as well.
Take three Peanut's Bake Shop Cookie Kits (which you can buy HERE), empty all of the mix into a large mixing bowl, and then add the amount of water and oil stated on the back of the packages. To clarify, since you're using three cookie kits, you'll need to triple the amount of water and oil stated on the back, as well. For instance, if you choose the original Chocolate Chunk flavor for this cake, you'll need three Chocolate Chunk Cookie Kits, 3/4 cup of oil, and then 1/2 cup + 1 Tb. water. Then - stir away!
Once you're at the point where you're staring at a big bowl of cookie dough, resist the temptation to eat it all and instead transfer that dough over to your greased pan. You'll then want to smooth over and flatten the surface using your hands or the back of a spatula.
Bake your mega cookie cake in your preheated oven for 24 - 26 minutes or until the edges start to turn a golden brown color. Once it's done, place it on a cooling rack to cool.
If you plan on decorating your cake with frosting, wait until it's completely cool before doing so.