Preheat the oven to 325 degrees convection bake (or 350 convention bake). Prep three 6" cake rounds with a swipe of shortening and a dust of flour to prevent sticking. Set aside.
In a food processor, pulse together the cookies and butter until a crumble forms. Split the crumble three ways between the three cake pans. Press down evenly with the back of a measuring cup or glass. Set aside.
In a medium bowl, whisk together the pumpkin, pumpkin pie spice, sour cream, eggs, oil, buttermilk, and vanilla. Hand whisk until everything is thoroughly combined. Sift in the cake mix and whisk until just combined (don't overmix!).
Bake for 32-35 minutes. Remove from the oven, let sit in the pan for 3 minutes, then flip out onto a wire rack to cool completely. I recommend wrapping them in plastic wrap, freezing them, and frosting the cake rounds when they are thoroughly chilled/frozen.