Steps
1. Preheat the oven to 350°F (180°C) and grease a 9-inch round cake pan.
2. In a large bowl, combine flour, sugar, cocoa, baking powder, baking soda, and salt. Add milk, oil, egg, and vanilla; beat on medium speed for 2 minutes.
3. Stir in boiling water to the cake batter (it will be thin). Pour into the prepared pan.
4. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Let the cake cool completely.
5. For the Oreo filling, crush some Oreo cookies and set aside. In a separate bowl, whip the heavy cream with powdered sugar and vanilla until stiff peaks form.
6. Fold in the crushed Oreo cookies into the whipped cream.
7. Once the cake has cooled, slice it in half horizontally. Spread the Oreo filling on the bottom layer and place the second layer on top.
8. Use the remaining filling to frost the top of the cake. Crush more Oreo cookies and sprinkle on top for garnish.
9. Refrigerate the cake for at least 1 hour before serving.