Using long serrated knife, split cakes horizontally into 3 equal layers. Place 2 bottom layers on serving plate, long sides together.
Cut two 1/2-inch-wide lengthwise slices of ice cream and position on the bottom cake layers on the serving plate. Place in freezer along with the remaining ice cream until hardened. Keep remaining cake layers covered.
Remove ice cream and cake layers from freezer. Top ice cream with 2 of the bananas, cut into thin slices. Drizzle with half the chocolate syrup and sprinkle with half the pistachio nuts.
Repeat layering, with cake, ice cream, syrup and nuts; fill in any spaces on ends of layers with additional strips of ice cream. Finish top with cake layers. Return to freezer until hardened.
About 15 minutes before serving the cake, beat together the heavy cream, confectioners sugar and vanilla in a small bowl with electric mixer on medium speed until soft peaks form. Spread the beaten heavy cream mixture with a spatula over all four sides of the cake and the top to completely cover.
Garnish the cake, if desired, with chopped pistachios and maraschino cherries.