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.