reheat the oven to 350°F/180°C. Butter the bottom and sides of one nonstick 9-in. round cake pan and set aside.
In a mixing bowl, sift together the flour, cocoa, baking soda and salt.
Combine the oil, sugar, egg and vanilla in the bowl of an electric mixer fitted with a flat paddle and beat on medium speed for 1 minute.
Reduce the speed to low and incorporate the cocoa mixture and the buttermilk in three additions, starting and ending with the cocoa mixture. Mix thoroughly and scrape the inside surface of the bowl after each addition. Mix an additional 10 seconds after the last addition.
Spoon the batter into the prepared pan. Place the pan on a baking sheet and bake for 30 - 35 minutes or until a toothpick inserted in the center comes out clean.
Cool on a wire rack for 15 minutes. Invert the cake onto a wire rack and cool completely.
While the cake is baking, break 10 - 12 cookies into small pieces and mix them into the softened ice cream. Spoon and pack the ice cream into a second nonstick 9-in. round cake pan and smooth the top.
Place a piece of plastic wrap directly on top of the ice cream and freeze for several hours or until the ice cream is firm.
To assemble the cake, transfer the cake layer to a plate or platter. Unmold the ice cream layer on top of the cake layer. Spoon the hot fudge sauce over the ice cream layer, spreading it into a smooth layer.
Garnish with additional cookie pieces if desired. Slice and serve the cake immediately.