For the cake: Preheat oven to 350 degrees. Line a 9-inch springform pan (that's at least 3-inches high) with heavy duty 18-inch wide aluminum foil (be sure not to tear foil, or you'll need to start over with a new sheet). Spray bottom of foil with non-stick cooking spray and lightly spray the sides.
In a large mixing bowl whisk together flour, 7/8 cup sugar, the cocoa powder, baking soda and salt.
Add sour cream, vegetable oil, egg, and 2 tsp vanilla extract. Using a wooden spoon or a spatula stir until well combined.
Pour mixture into prepared pan and spread even. For a perfectly level cake you can wrap a wet cake strip around the pan before baking. This is optional but helpful.
Bake in preheated oven until toothpick inserted into center comes out clean, about 25 to 30 minutes.
Cool completely on a wire rack, about 2 hours.
For the fudge sauce: Spread the fudge sauce evenly over the cake layer. Freeze 1 hour. During the last 30 minutes remove the ice cream from the freezer and set in the fridge to soften.
For the ice cream: Scoop slightly softened ice cream out and dollop across the cake over the fudge layer. Gently press down to level and spread even, while filling in any spaces. Freeze 2 hours until firm.
Remove the cake from the freezer, while tilting the pan quickly run warm water around the outside ring of the pan (don't get cake wet!), then loosen the springform pans ring. Transfer to a platter with ice cream layer upright (while removing the foil) and return to freezer.
For the whipped topping: In a large mixing bowl combine the heavy cream, 3 Tbsp sugar and 1 tsp vanilla.
Using an electric hand mixer set on medium-low speed whip mixture until it starts to thicken slightly, then increase to high speed and whip until very stiff peaks form.
Spread mixture over cake. You can spread over just the top, or the top and the sides. You can also pipe swirls of whipped cream around the top edge of the cake if desired and add sprinkles or finely chopped chocolate for decoration.
Freeze until it's starting to firm up, about 1 hour, then wrap with foil to prevent freezer burn and freeze 1 hour longer, or until firm. Cake can be frozen for 2 weeks.
Cut into slices and serve quickly after slicing.
*Be sure to use a thick fudge sauce otherwise a runny fudge sauce will be a mess and will just soak into the cake and make it soggy.
Other delicious flavors of ice cream to use here include mint chocolate chip, cookie dough, chocolate, caramel (not salted or it's too soft), peanut butter, cherry, coffee, cheesecake, pistachio or vanilla bean.