Whether it be made from scratch or produced from a box of premixed ingredients, cake batter requires a certain amount of air as it is the air in the batter that serves to give texture and volume to a finished cake. Too much air, however, produces "tunnelling" or pockets of air that lead to a finished product with large holes throughout and splitting layers. A few simple steps taken before and during the baking process can help prevent tunnelling and even lower the fat content of a cake, producing a level, fluffy cake with the proper volume, appearance and consistency.
Sift all dry ingredients together so that they are evenly distributed, especially the leavening agent (baking powder or baking soda). This aerates the batter while also ensuring that there are no lumps in the batter. Should the leavening agent not be evenly distributed, tunnelling is more likely to occur.
Ensure that the batter's ingredients are at the proper temperature and that you do not overly mix the batter. Air bubbles are needed in the batter to give the finished product the proper consistency. However, overly mixing the batter will damage the air bubbles that the batter needs in order to give the cake proper volume. Likewise, ingredient that are left to melt before the cake enters the oven, such as butter, will not interact properly with the other ingredients and will alter the consistency of the batter, leading to a flat, uneven cake.
Substitute the butter in your recipe for vegetable or canola. The high butterfat in butter tends to produce more air bubbles. This will also produce a healthier -- albeit a slightly denser -- finished cake.
Pour the batter into the pan. Lift the pan about a foot from the counter. Drop the pan onto the counter three or four times. This will force the air in the batter to bubble to the surface while also serving to level your batter. Finish by swirling the batter so that it climbs the sides of the pan as this will help prevent the tops of the cake layers from splitting.
Immediately place the pan in the oven. Letting the batter sit will allow the baking powder or baking soda in the batter to activate prematurely. The role of baking powder/soda in a cake batter is to enlarge the air bubbles created during the creaming of fat and sugar. This is what leads to a level cake with ideal volume, but this must happen at the proper time during baking.
Open the oven halfway through the baking process and tap each side of the pan with the butt end of a kitchen knife. This will burst large bubbles in the batter and allow the batter to settle before the cake has time to permanently rise and set.