Preheat oven to 325 degrees. Spray 3, 6 inch rounds with cooking spray, put parchment paper round at bottom of pan then spray again with cooking spray or place cupcake liners in cupcake pan. Set aside.
Whisk together pumpkin, pumpkin pie spice, eggs, sour cream, buttermilk, oil, and vanilla.
When combined, sift in cake mix and stir until just combined. Do not over mix, but make sure all the dry ingredients have been incorporated.
Divide cake batter evenly between cake pans or (24) cupcake liners.
Bake cake layers for 30-35 minutes, or 14-16 minutes if you're making cupcakes, until center is baked all the way through. Do not over bake!
Let cool in pans for 10 minutes then flip out onto wire cooling rack until cakes are room temperature, if making cupcakes only leave them in the pan for a few minutes then move to cooling rack.
Wrap cake layers in plastic wrap and put in freezer. Stacking and assembly is SO much easier with frozen cakes!