The pastry shop where I work uses a chiffon-type recipe with egg whites folded in at the end. The "rubber-bottom" problem seems to occur when the eggs are underbeaten or badly overbeaten and the batter sits too long before the pans go into the oven. This can be a matter of seconds... When you see large bubbles (>1 cm) forming in your batter, the proteins supporting the egg foam are starting to denature (chemical changes from sugars, fats and acids in the batter, physical breakage of the protein chains from beating). [***A quick eyeball test for properly beaten whites --- watch the sides of the mixing bowl at the top of the foam layer. When the billows of egg foam start to pull tiny peaks away from the sides of the bowl, it's at the soft peak stage. When the tips of the peaks on the sides of the bowl stay horizontal, it's at the stiff peak stage.***] Good mise en place is essential! Before you fold in your egg whites, make sure that you have your pans prepared and ready, your oven at temperature, and keep any spatulas you need to level your pans immediately at hand. For folding in your egg white, try using a bowl scraper --- as someone else here suggested, roll up your sleeves and fold bare-handed. Add the beaten whites all at once, unless the batter is very stiff and you have to moisten it with a portion of the whites to make the rest "foldable". It's more important to get the whites incorporated rapidly and thoroughly than it is to be gentle with them. Fill your pans and get them into the oven quickly --- don't worry about trying to even up your layers by shuffling spoonfuls among pans. You can trim them to match after they're baked. [***If you make the recipe repeatedly or have several equal-size pans to fill, sit down and add up the weight of your recipe before you start cooking, then divide by the number of pans and use a kitchen scale to portion your batter. It's more accurate and much faster! If you write down the weight per pan of each size, you can use this information in the future to adjust your recipe for the number of layers you need to bake.***] Finally, cakes that leaven with egg white foam are notorious for shrinking when the elastic egg proteins contract after the layers are removed from the oven. The domed center of the cake layer is the most "stretched" part, so it leaves that hollow spot as it shrinks and may also pull the cake away from the sides of the pan. Try inverting your cake pans onto sheets of parchment or Silpats immediately after you remove them from the oven. You can peel off the parchment/Silpat when the layers are cool. The outermost skin of the layer may come off, but it's a small price to pay for a well-shaped cake. Sorry for the long-winded response, but most cookbooks don't go into the details of technique or the hows-and-whys, and sometimes they should. Best of luck in your future cake-baking endeavors! ~Miriam
