Yes, you can use a plant-based yogurt in place of Greek yogurt and a flax egg in place of the chicken egg. Dark chocolate is typically vegan, but be sure to double-check the ingredients list to be sure. To make a flax egg, stir together one tablespoon flaxseed meal (not the whole flaxseeds) and 3 tablespoons water. Let the mixture rest for about 10 minutes, or until it becomes thicken and gelatin-like.