The best substitute for butter, in terms of taste and texture, is margarine. Margarine lends a small amount of that rich and creamy “butter” taste to the molten lava cake. Shortening, which is basically flavorless, also works in a pinch. You can use margarine or shortening cup for cup for butter. If you’re looking to cut down on fat, you can replace up to half the butter in the molten lava cake with applesauce. However, you do need around half the fat to remain for the cake to bake properly.