Lightly butter 9-inch springform pan.

Spread chocolate ice cream into an even layer in the bottom of the springform pan. Freeze for at least 30 minutes.

Spread hot fudge sauce over chocolate ice cream then top with crushed Oreo cookies and press lightly so that they adhere to the hot fudge. Freeze for at least 30 minutes.

Add vanilla ice cream and spread it into an even layer. Freeze for at least 8 hours.

At least 1 hour before serving, remove the cake from the springform pan. To release the cake from the pan, invert cake onto a plate and wrap a hot kitchen towel around the cake pan until the cake loosens. Slide pan off. Remove the bottom of pan.

Top with a serving plate and turn the cake right side up. Return to the freezer for at least 20 minutes.

Make the whipped cream. Combine heavy cream, sugar, and vanilla extract in the bowl of a stand mixer. Whip on medium-high speed until you can clearly see the whip lines in the cream.

Pour water into a small microwaveable bowl then sprinkle unflavored gelatin on top. Let stand until thickened. Microwave 1 minute until gelatin has melted. Stir well to make sure no lumps remain.

Allow gelatin to cool slightly or add about 1/2 cup of cold heavy cream to cool the mixture. With the mixer on low, slowly pour in the gelatin mixture in a continuous stream. Increase mixer speed to medium-high and whip until stiff peaks form.

Spread half the whipped cream on the top and sides of the cake. If ice cream starts getting soft, place it back in the freezer for a few minutes to firm up.

Decorate with additional whipped cream and sprinkles if desired.