Some cakes actually do really well with olive oil instead of butter. It will give it a bit of a different flavor and texture, but not overwhelmingly so! I actually think olive oil complements citrus well -- it's why you see so many citrus vinaigrettes. I have not tried using it in this recipe, but if you want to give it a go, be sure to use 3/4 of the amount of butter you'd need. So, if you need 8 tablespoons of butter (1 stick), only use 6 tablespoons of olive oil.