Line an eight- inch springform pan with plastic wrap so that it extends all the way up the sides and beyond by a few inches.
Scoop the chocolate ice cream out into a big bowl and stir it until it's soft enough to be placed in the pan and flattened into an even layer. Sprinkle on the chocolate cookie crumbs. Cover the ice cream with plastic wrap and freeze it for one hour. Also, you may want to wrap the bottom of the pan in aluminum foil, in case of drips.
Scoop the vanilla ice cream into the bowl, and stir in the chopped chocolate bar. Smooth the ice cream in an even layer, right on top of the chocolate crumbs. Cover with plastic wrap and freeze for six hours or overnight.
After the six hours, make the ganache: Heat the cream in a two quart saucepan, just until it begins to simmer. Pour it over the bittersweet chocolate, and stir until it's totally smooth. Let it cool to room temperature.
Remove the sides from the cake and the plastic wrap and invert it on to a serving plate. Spread the cooled ganache all over the top, letting some of it drip down the sides. Refreeze for a couple more hours. Add the sprinkles around the perimeter of the cake.
In an electric mixer, beat the cream and sugar together, until it's whipped together. Put it in a pastry bag with a star tip attached and pipe swirls around the top of the cake. Add more sprinkles and serve with extra whipped cream and sprinkles.