Heat oven to 350ºF. Grease a rimmed 13-x-18-inch baking sheet and line the bottom with parchment. Spread batter in pan and bake cake until done, about 40 min. Cool, then turn out on a wire rack and remove parchment. (Chill the cake overnight if possible for easy handling)
Cut cake to make 3 equal pieces measuring about 4½ by 11 inches each; discard cake scraps or save for another use. Cut the 3 layers in half horizontally to make 6 very thin and even cake layers.
On a serving platter top one thin cake layer with about ⅓ cup Whipped Chocolate Ganache and spread in an even layer. Repeat with remaining 5 layers. Frost the entire outside of the cake, smoothing the sides and top.
The Kit Kat bars are shorter than the height of the cake, so pipe a few rows of dots of ganache around the base of the cake with a #10 or #12 plain piping tip. Decorate the edges with the Kit Kat bars and the top with extra sliced bars. Chill until ready to serve.
Notes
Chilled cake is always easier to assemble. If you've got time, bake the cake a day ahead and keep it wrapped in the fridge until you're ready to use it.