There are several possible reasons why a cake may become hard after baking:
Overbaking: One of the most common reasons for a cake to become hard is overbaking. If a cake is left in the oven for too long or at too high a temperature, it can result in excessive moisture loss, leading to a dry and hard texture.
Incorrect measurements or ratios: Baking is a science that requires precise measurements. Using too much flour or not enough liquid in the cake batter can cause the cake to become dense and dry.
Too much leavening agent: Using too much baking powder or baking soda can cause a cake to rise too quickly and then collapse, resulting in a dense and compact texture.
Lack of moisture: Moisture is essential for a tender and moist cake. If the recipe doesn't include enough liquid or fat (such as butter or oil), or if the cake is baked for too long, it can lead to a dry and hard texture.
Improper mixing: Overmixing the batter can lead to excessive gluten development, which can result in a tougher texture. It's important to mix the ingredients just until they are combined and no further.
Low-fat content: Cakes that are low in fat content tend to be drier and harder. Fat helps create a tender texture by coating the flour particles and preventing gluten formation.
To avoid a hard cake, it's important to follow the recipe carefully, measure ingredients accurately, bake the cake for the recommended time and temperature, and ensure the batter has adequate moisture and fat content. Checking for doneness with a toothpick or cake tester inserted into the center of the cake can help prevent overbaking.