Preheat oven to 350 degrees F (176 C). Line a 9" round cake pan with foil leaving enough to overhang around the edges. This will help you to remove it from the pan later.
In a large bowl combine the oil, sugar, eggs, and vanilla. Add the flour, cocoa powder, and salt. Mix just until combined and no more. Pour into prepared pan. Bake for 30-35 minutes or just until edges are set and a toothpick comes out with moist crumbs. (The toothpick should not be wet with batter). Let cool completely in pan.
Let ice cream soften so it's easier to work with. Meanwhile, Line another 9" round cake pan with plastic wrap. Be sure to leave plenty of overhang to cover ice cream with.
Once ice cream is soft enough spoon into prepared pan and smooth into an even layer. Try to press out any air pockets. Cover with plastic wrap and freeze for 8 hours and up to 3 days.
Once ready to assemble remove brownie from pan and remove foil. Place on desired cake stand or plate. Spread hot fudge sauce over top of the brownie.
Remove ice cream from pan and remove plastic wrap. Quickly place on top of hot fudge sauce. Cover cake with plastic wrap and place back in the freezer for 30 minutes.
Meanwhile, in the bowl of a stand mixer whip the cream, powdered sugar, and vanilla until stiff peaks form.
Remove cake from freezer and spread whipped cream all over the ice cream and brownie. You can pipe decorations if you like! Place in freezer for 30 minutes or until whipped cream is solid.