Cake mixes that bake up light and fluffy often include less fat and sugar and greater amounts of baking powder. Less fat and sugar mean a reduction in the weight, or denseness of the cake, while the leavening properties of baking powder increase the volume of a cake mix as it bakes. To thicken or dense up a cake mix, you need to make adjustments both in the ingredients you use and the procedures you follow when the preparing the cake batter.
Substitute equal amounts of heavy ingredients for lightweight ingredients. For example, in a cake mix that calls for 1 cup of water, substitute plain or flavored yogurt, or milk that contains some amount of milk fat. The greater the percentage of fat, such as you find in whole milk or buttermilk, the denser the cake will be. Substituting whole eggs for egg whites will also increase density.
Add ingredients that will increase density but that the original cake mix recipe does not call for. A common example is adding a 1 oz. package of any flavor of instant pudding and one whole egg or two egg yolks to your cake-mix batter.
Mix the cake according to the instructions your recipe provides. This often means using a hand mixer set to low speed for about 30 seconds to moisten the ingredients, then to medium speed for no more than two minutes to blend the ingredients. Avoid over-mixing cake batter. The longer you continue mixing the more air you incorporate into the batter and the lighter your cake will become.
Pour less cake batter into each pan. Fill each pan with batter to a depth of no more than about 1 inch. Use additional pans if necessary. For example, make a three-layer, rather than a two-layer, cake, or fill a small cupcake tray with any extra batter.