What can i use instead of butter for a cake?

You can use any other oil or lard. Cream and other milk products may contain enough fat to have much the same effect as the butter on the texture and taste of your cake. As for fat-free options you could search for and experiment with vegetable gums such as Guar Gum, Pectin, Xantham Gum, etc... that are used in industrial baking as fat substitutes.