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1
Preheat oven to 350. Grease an 8-inch round cake pan.
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2
Whisk together sugar, both flours, cocoa, baking soda and salt. Gradually whisk boiling water into flour, adding about 1/2 cup of water at a time. Whisk in eggs one at a time, then oil.
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3
Bake 50 to 55 minutes. Let the cake cool in the pan for 30 minutes, then invert onto a wire rack.
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4
Spray an 8x3-inch round cake 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 adhere.
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5
Trim off extra cake to get a smooth top. Place the cake in the bottom of the prepared cake pan for the base.
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6
Allow ice cream to soften in the refrigerator until it's slightly softened and spreadable. Spread ice cream all the way to the sides of the pan, filling any gaps or holes. Put the assembled cake in the freezer as soon as it's filled and freeze overnight.
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7
At least 1 hour 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. Top with a serving plate and turn the cake right side up. Return to the freezer for at least 10 minutes.
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8
Make the whipped cream. Combine the heavy cream, sugar and vanilla. Whip on medium-high speed until the cream is light and fluffy and holds a medium to firm peak, 2 to 3 minutes.
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9
Spread the whipped cream on the cake. (If it gets soft, just pop it back into the freezer for 5 minutes.) Use the Fudge Sauce to pipe borders. 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.
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10
Remove the cake from the freezer 5 minutes before serving to soften it slightly.
