In most banh mi sandwiches, you will find both butter and mayonnaise. The two ingredients work together to add creaminess and flavor to the sandwich. Some recipes call for only one or the other. Still, typically you will find both used in traditional Vietnamese banh mi recipes.  Butter adds a rich flavor, while mayonnaise adds tanginess and helps to hold all the ingredients together. Together they make a delicious combination that packs a flavorful punch!  So if you're wondering what's inside the traditional banh mi sandwich, the answer is butter and mayo. Enjoy!