Prepare cake mixes and bake according to directions (any size pans).

You should also prep the pudding mix and chill it until its needed again.

Crumble the white sandwich cookies in small batches in a food processor, scraping often. Set aside all but about 1/4 cup. To the 1/4 cup cookie crumbs, add a few drops green food coloring and mix until completely colored.

Once the cakes are finished baking, and cooled to room temp, crumble them into a large bowl (the cakes, i mean). add HALF of the remaining crumbled vanilla cookies. Next, mix in the pudding. Don't add to much: you dont want the cake to be soggy, just moist. Combine gently.

Line a new, clean kitty litter box. Put the cake/pudding/cookie mixture into the litter box.

(sorry no picture here as the last time i made the cake i forgot to take some!)

Put 3 unwrapped tootsie rolls in a microwave safe dish and heat until soft. Shape ends so they are no longer blunt, curving slightly. Repeat with 3 more Tootsie rolls and then bury them in the mixture. Sprinkle the other half of cookie crumbs over top. Scatter the green cookie crumbs lightly on top of everything as this is supposed to look like the chlorophyll in kitty litter.

Heat 3 more tootsie Rolls in the microwave until almost melted. Scrape them on top of the cake and sprinkle with cookie crumbs. Spread remaining Tootsie Rolls over the top. For the finishing piece, take one tootsie Roll and heat until pliable, hang it over the side of the kitty litter box, sprinkling it lightly with cookie crumbs.

(once again, no picture yet, sorry)

Some other tips:
since you cover the top of the cake with crumbs anyways, it doesn't really matter what type of cake you use so dont feel like you have to use white and chocolate cake.

also, you can also opt not to crumble the cakes, but rather layer them in the pan with the layers of pudding in between. sprinkle the top layer of pudding with a heavy layer of crumbled cookies. Same effect, but a different texture.

(here's the finished product again)