To make these muffins vegan, you can use a flax egg in place of the egg. Just mix together 1 tablespoon of ground flax seed with 2 1/2 tablespoons of water and let it sit for at least 10 minutes. The mixture is ready to use when it is slightly thickened. Note that while the muffins still turn out moist and fluffy, I found that using a flax egg resulted in a slightly thicker batter. You'll also want to ensure that your chocolate chips are vegan.