Adjust oven rack to middle position and preheat oven to 350°F (180°C). Line bottoms of two 9-inch cake pans with parchment paper and lightly coat the insides with non-stick pan spray.

Sift flour, cocoa, baking soda, and salt into a large bowl; set aside.

In a medium bowl, whisk together sugar, sour cream, oil, eggs, and vanilla until smooth.

Whisk wet ingredients into dry ingredients until smooth. Divide batter evenly between prepared cake pans.

Place pans together on one oven rack and bake until cake is just firm and toothpick inserted into center comes out with moist crumbs, about 25 minutes.

If oven has uneven heat, pause to rotate the pans after about 12 minutes.

Let cakes cool in pans for 15 minutes, then run a butter knife around the edges to loosen. Invert onto a wire rack, peel off the parchment, and return cakes right side up. Cool cakes completely on wire rack, about 1 hour.

Carefully cut 6 Oreo cookies in half; set aside. Chop remaining cookies into ¼-inch pieces; set aside.

In the bowl of a stand mixer fitted with whisk attachment, whip 2 cups cream on medium-high speed until thick and fluffy. Transfer to a large bowl and place in refrigerator.

In the same mixer bowl, whip remaining 2 ½ cups cream, sugar, and vanilla until thick and fluffy. Using a flexible spatula, fold sweetened whipped cream into already whipped cream.

Transfer 1 cup whipped cream to a small bowl and refrigerate until ready to decorate cake. Fold chopped Oreos into remaining whipped cream.

Level cakes with a serrated knife and set scraps aside for snacking. Place one layer on a heavy cast iron turntable.

Top with about one-third of the Oreo whipped cream, using an offset spatula to spread it evenly from edge to edge.

Top with second layer and then cover the top and sides of the cake with remaining Oreo whipped cream, spreading it as smoothly as you can.

Refrigerate cake to let cookies soften, about 2 hours.

Re-whisk reserved whipped cream and transfer to a pastry bag fitted with star tip. Pipe 12 whipped cream rosettes around perimeter of cake and garnish with reserved Oreo cookie halves. Serve.