Coat the bottom and sides of a 9-inch round springform pan with non-stick cooking spray. Line the sides and base with waxed paper – the spray will help it adhere to the pan. Trim the paper to the height of the pan.
Finely crush the Oreos in a food processor. Add the milk and blend again until the mixture holds together. Set aside.
Cut each rainbow cookie into thirds. Place one layer of cookies in a ring around the inner side of the cake pan, packing them snugly together. Repeat with the remaining cookies, stacking them up, to create a tightly-packed edge.
Spoon the oreo cookie crumbs into the centre of the pan and press firmly to cover the bottom. Freeze for 1 hour.
Remove all 3 flavours of ice cream/sorbet from the freezer. Let them soften for 10 minutes at room temperature. Scoop them out into individual bowls and beat until they are a soft but spreadable consistency.
Remove the cake pan from the freezer. Scoop the raspberry sorbet into the cake pan, spreading it out into an even layer. Top with the vanilla ice cream in an even layer. Lastly spread the mint chocolate chip ice cream over it in an even layer. Place in the freezer while you whip the cream.
For the chocolate whipped cream:
In a medium bowl, place the chilled heavy cream, sugar and cocoa powder. Use a whisk or electric beaters to whisk until thickened to soft peaks. Transfer to a piping bag fitted with a star tip.
Remove the cake from the freezer. Pipe the chocolate whipped cream over the top of the cake in little swirls. Place the maraschino cherries on top of the whipped cream around the edge of the cake.
Freeze the cake overnight until solid.
To serve, unclip the sides of the pan and pop the cake out. Peel away the waxed paper from the sides. Let sit for 15-20 minutes to soften. Use a sharp knife, dipped in hot water and wiped clean between cuts, to slice the cake into wedges and serve.