Evenness is the key to a properly stacked layer cake: The batter needs to be spread to an even height, without peaks or valleys, and each band of frosting needs to be uniform in depth. The best tool to get those even layers of batter and frosting (and one of the most indispensable tools for baking, period) is a small offset spatula, a thin strip of metal set below its handle by an inch. In bakeries and restaurants, larger offset spatulas (both in length and width) are used for smoothing giant sheet pans of batter and spreading large swaths of frosting, while smaller ones are used for more detailed decorating. I find the large ones clunky when making a single cake at home and prefer the smaller spatula for the task, allowing me to spread cake batter evenly in the pan and apply a consistent coat of frosting with precision. Williams-Sonoma makes a beautiful yet utilitarian version (4-inch-blade spatula, $13 at williams-sonoma.com).