Several ingredients, such as extra butter, water, oil, sour cream, egg yolks and milk or buttermilk, can make your butter cake more moist. Adding more moist ingredients, along with more gentle mixing and careful attention to baking time, helps retain moisture in cake batter.
The biggest variable in baking butter cake is the moisture content in flour because the drier flour is, the more moisture it soaks up. Compensate for this by adding or increasing moist ingredients. One easy way is by adding more butter or egg yolks to your cake batter. About two tablespoons of butter or two large egg yolks can make a cake moister. You can also add and retain moisture by adding about 1/2 a cup of clean vegetable oil. To enhance flavor while adding moisture, add about the same amount of sour cream, buttermilk or regular cow's milk. To add moisture without adding calories or saturated fat, add about 1/2 cup of water. Should the batter be too loose, adding about a teaspoon of flour will re-balance it.
Along with adding moist ingredients, you can keep a cake batter moist by not over-stirring as you mix ingredients. This improves the texture as well. Remember to grease and flour cake pans, and to test for doneness by inserting a wooden toothpick into the center of each layer. The cake is done when the toothpick comes out clean.