Lightly grease the bottom and sides of a 9-inch springform pan. Line the bottom of the pan with parchment paper.
In a stand mixer fitted with a paddle attachment, churn the chocolate ice cream until it becomes soft and spreadable.
Using a silicone spatula, spread the softened chocolate ice cream evenly in the bottom of the springform pan. Place the springform pan in the freezer for about 30 minutes.
While the chocolate layer hardens, pour the hot fudge sauce into a medium bowl. Stir the sauce to loosen it for easier spreading. Set the bowl aside.
Separate the cream from the 12 Oreo cookies. Finely chop the Oreo wafers. Discard the cream. Set the chopped wafers aside.
Remove the springform pan from the freezer. Spread the hot fudge topping over the chocolate ice cream. Sprinkle both the finely chopped Oreo cookies and the Heath Bits O' Brickle over the hot fudge topping. Place the pan back in the freezer for about 30 minutes.
While the springform pan is in the freezer, churn the vanilla ice cream in a stand mixer fitted with a paddle attachment. Churn until the ice cream becomes soft and spreadable.
Remove the springform pan from the freezer. Using a silicone spatula, carefully spread the softened vanilla ice cream on top of the Oreo cookie and Heath Bits O' Brickle layer. The vanilla ice cream layer will be slightly more difficult to spread, since the hot fudge topping will not be completely frozen.
Cover the springform pan with tinfoil. Freeze for at least 8 hours.
Before you release the cake from the springform pan, you must chill the large plate and the cake stand for about 5 minutes. This will keep the ice cream cake from melting when it makes contact with them.
Remove the springform pan from the freezer. Turn the pan upside down over the chilled large plate. Wrap the pan in a warm kitchen towel. The towel will help loosen the ice cream cake from the pan. Gently push down on the base of the pan to slide the cake out of the pan and onto the plate. Discard the parchment paper. Immediately place the large plate with the cake on it into the freezer for about 15 minutes.
Remove the ice cream cake from the freezer. Top the cake with the chilled cake stand and turn the cake and stand right side up. Place the cake stand with the cake on it into the freezer for at least 15 minutes.
In a stand mixer fitted with a whisk attachment, whip the heavy whipping cream, powdered sugar, nonfat powdered milk, and vanilla extract at medium-high speed until the cream is light and fluffy. Remove the cake from the freezer.
Working swiftly, use a long offset spatula to top the center of your cake with the frosting. Carefully spread the frosting evenly just past the edges of the cake. Use the overhang of frosting to coat the sides of the cake.
Place the rest of the frosting into a disposable piping bag fitted with a Wilton 21 piping tip. Pipe a border around the top and bottom edges of the ice cream cake. Add the rainbow sprinkles to the top of the cake. Place the cake back in the freezer for about 30 minutes.
Remove the cake from the freezer. Prepare a hot knife to cut slices of the ice cream cake easily. Dip your knife in a glass of hot water or run it under a hot faucet. Wipe the blade clean with a kitchen towel, and cut a slice of the cake. Repeat for each slice. Enjoy!
The homemade whipped cream frosting yields about 4 cups.
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