While using vegetable oil is a healthier option than butter in a store-bought cake mix, you can substitute one for the other if your bottle of oil is empty. Just replace the vegetable oil with melted butter. Don't be surprised, however, if the flavor and consistency of your dessert is slightly different than what you're used to. Some may like the more noticeable dairy flavor of the cake, while others will yearn for the moistness that oil provides.
Good Fats vs. Bad Fats
While butter and vegetable oil are both fats, their composition is very different. Butter is made from animal fats and contains 80 percent fat, along with water and milk solids. Vegetable oil is derived from plants and is made up of 100 percent fat. Butter primarily contains saturated fat, while vegetable oil, like canola oil, is very low in saturated fat and higher in polyunsaturated and monounsaturated fat. It's the saturated fat that makes butter unhealthy, because it can raise your low-density lipoprotein cholesterol, commonly referred to as your "bad" cholesterol.
Butter as the Pinch Hitter
When swapping out butter for vegetable oil in a cake mix, melt the butter first in the microwave so that it is liquefied. Do this in 15-second increments to avoid burning the butter, which can negatively affect its flavor. Use either the same amount of butter or about 20 percent more than the directions call for to make up for the difference in the fat content between butter and vegetable oil. If the mix calls for 1 cup of vegetable oil, then substitute 1 cup of melted butter and just under a 1/4 of a cup of melted butter.
I Can Believe That It's Butter
Cakes made with butter instead of vegetable oil not only contain more saturated fats, they also have more calories. In addition, the taste and texture differs from cakes made with oil. Cakes made with butter have more of a dairy flavor from the milk solids and a richer but more crumbly texture. Oil adds more of a neutral flavor but a moister texture to cakes. Use unsalted butter if possible, because the salt in the butter may also affect the flavor of the finished cake. Note that melting the butter is important so you don't wind up with small flecks of solid butter throughout your batter.
Exploring Your Options
While butter can stand in for oil in a cake mix, it isn't the only substitute available. Substitute applesauce for the oil in an equal amount. Applesauce does not add any fat and is actually healthier than the oil. Or use melted margarine in an equal amount or around 20 percent more, just as with butter. Use an unsalted margarine that is low in saturated fat and lacks trans fats. As with butter, the texture of the cake may not be quite the same as with oil.