Boxed cake mix is amazing because it usually only calls for two additional ingredients to be able to whip up an easy yet decadent cake in a flash: eggs and oil. The oil is what makes it super moist, but subbing in butter as the fat can make it taste even richer.
Measure out an equal amount of butter as the amount of oil called for in the recipe, and put the butter in a small, microwave-safe bowl.
Place the bowl in the microwave and heat for 10 seconds. Then stir the butter lightly and microwave again for 10 more seconds. Repeat until the butter is completely melted.
Follow the instructions on the cake-mix box, and add the melted butter in place of the oil. Mix the batter until smooth.
Bake the cake according to the instructions on the box. The cake is done when a toothpick inserted into the middle comes out clean.