I don't know how that would work with the milk and whether you should substitute both the oil and milk with butter or leave the milk. I never tried this specific recipe with butter before, but maybe try it and see what happens.
BTW you can substitute the milk with a cup of yoghurt.
I did one with yogurt before and it ended up pretty soft, I liked it. Maybe I'll try it with butter and that instead... Which by now is a whole new recipe