Preheat the oven to 350°F

Prepare cake mix according to package directions using buttermilk, oil, and eggs. Portion the batter into a greased or sprayed 9-by-13-inch baking pan. Bake according to package directions. Let cool completely. Once cool, cut about half of the cake into 1-inch pieces for roughly 6 cups of cubes. Save the remaining cake for another use.

In a stand mixer, paddle the room-temperature cream cheese and blend until smooth and creamy.  Scrape down the bowl and add the pumpkin puree, 1 cup of sugar, pumpkin pie spice, and 2 tsp vanilla extract. Beat until fully incorporated, scraping down the bowl as needed.

With a clean bowl and the whisk attachment, whisk the heavy cream, 1/2 cup powdered sugar, and remaining 2 teaspoons vanilla extract on medium-high speed until stiff peaks form, about 1 1/2 minutes. Carefully fold 1 1/2 cups of the whipped cream into the pumpkin mixture.

Arrange half of the cake pieces in the bottom of a large trifle dish. Spread half of the pumpkin mixture on top of the cake evenly. Sprinkle half of the crushed cookies on top of the pumpkin, followed by half of the whipped cream Repeat layers and garnish with cookie crumbs. Serve immediately, or cover and chill for up to 1 day before serving.