Preheat oven to 350°F (175°C). Line a 9-inch round cake pan with foil and spray with nonstick cooking spray. You can also use a springform pan (grease it with cooking spray, no foil needed).

In a large bowl, whisk together flour, sugar, cocoa powder, salt, and baking soda. In a separate measuring cup or bowl, whisk together egg, vegetable oil, buttermilk, white vinegar, and vanilla extract until combined. Add the wet ingredients to the dry ingredients and mix with a hand mixer until smooth. Stir in the red food coloring until just combined.

Pour the red velvet cake batter into the prepared cake pan.

Prepare the cheesecake layer: In a medium bowl, beat cream cheese until smooth. Beat in sour cream, granulated sugar, egg, and vanilla extract until well combined. Gently stir in the crushed Oreo cookies. Carefully spread the cheesecake mixture over the red velvet cake batter in the pan.

Bake for 30-40 minutes, or until a toothpick inserted into the cake portion comes out clean. The cheesecake filling may still be slightly jiggly. Let the cake cool to room temperature, then chill in the refrigerator for at least 2 hours before frosting.

Prepare the frosting: In a large bowl, cream together softened cream cheese and butter until fluffy. Beat in the cocoa powder. Gradually add the powdered sugar, salt, and vanilla extract, mixing until smooth. If the frosting is too thick, add milk 1 teaspoon at a time until desired consistency is reached. Optionally, stir in more crushed Oreos.

Frost the edges of the cake and top with Oreo cookie halves for garnish.