For the Ice Cream:
2 quarts cookies and cream ice cream (that's a double-batch if using the homemade recipe)
For the Cake:
1½ cups granulated sugar
¾ cup all-purpose flour
¾ cup cake flour
½ cup Dutch-processed cocoa powder, sifted
1 teaspoon baking soda
½ teaspoon kosher salt
1½ cups boiling water
½ cup canola oil
For the Whipped Cream:
1 cup heavy cream
2 tablespoons granulated sugar
1 teaspoon vanilla extract
For the Decorations:
Hot Fudge Sauce
1. Two Days Before: Make a double batch of the cookies and cream ice cream base and chill.
2. One Day Before: Make the Cake. Preheat the oven to 350 degrees F. Grease an 8-inch round cake pan and line the bottom with parchment paper.
3. In a mixing bowl, whisk together the sugar, both flours, the cocoa, baking soda and salt. Gradually whisk the boiling water into the flour, adding about ½ cup of the water at a time, until all the water is added and you have a smooth, thick batter. Whisk in the eggs one at a time, blending well after each addition, then whisk in the oil. You should have a very thin but smooth batter.
4. Pour the batter into the cake pan. Bake until the cake springs back to a light touch and a toothpick inserted into the center comes out clean, 50 to 55 minutes. Let the cake cool in the pan for 30 minutes, then invert onto a wire rack. Remove the parchment and let cool completely.
5. Prepare the Cake Pan: Spray an 8x3-inch round cake pan (or springform/cheesecake pan) with nonstick cooking spray. Line the bottom of the pan with a round of parchment. Then cut two strips of parchment that are 3 inches wide by 14 inches long. Line the sides of the pan with the parchment strips, overlapping them slightly. spray a bit of nonstick spray where the ends overlap to help them adhere.
6. Prepare the Cake Base: If your cooled cake has a domed top, trim off the domed portion with a long serrated knife to achieve a level surface. Place the cake in the bottom of the prepared cake pan for the base of your ice cream cake.
7. Churn the Ice Cream: Churn the ice cream base according to your machine's instructions and transfer it directly from the machine into the cake pan. If your ice cream has been stored in the freezer (or if you're using store-bought), allow it to soften in the refrigerator for 20 to 30 minutes, until it's slightly softened and spreadable. Use an offset spatula to nudge the ice cream all the way to the sides of the pan, filling any gaps or holes, including any space between the cake and the pan. Rap the filled cake pan sharply on the counter a few times to encourage any air bubbles to rise to the surface and smooth the top. Put the assembled cake in the freezer as soon as it's filled and freeze overnight.
8. The Morning Of: At least 1 hour (and no more than 12 hours) before serving, unmold the cake. To release the cake from the pan, invert the cake onto a plate or cardboard round the same size and wrap a hot kitchen towel around the cake pan, keeping it there for 30 seconds, or until the cake loosens and slide the pan off. Remove the parchment (and bottom of pan if using a springform). Top with a serving plate and turn the cake right side up. Return to the freezer for at least 10 minutes.
9. While the cake is in the freezer, make the whipped cream. Combine the heavy cream, sugar and vanilla extract in the bowl of a stand mixer. Using the whisk attachment, whip on medium-high speed until the cream is light and fluffy and holds a medium to firm peak, 2 to 3 minutes.
10. Spread the whipped cream on the cake, working quickly so that the ice cream doesn't melt. (If it gets soft, just pop it back into the freezer for 5 minutes.) Use the Hot Fudge Sauce to pipe borders, if desired. Pipe some extra whipped cream on top and decorate with extra Oreo cookies. Covered with foil, the cake will keep in the freezer for 2 days.
11. 5 Minutes Before: Remove the cake from the freezer 5 minutes before serving to soften it slightly. A hot, dry knife will yield the cleanest cuts. Dip your knife in a glass of hot water, wipe it clean with a towel, and cut a slice. Repeat between every slice.