Mustard flour is just another name for mustard powder or dry mustard, which is composed of ground mustard seeds. Mustard flour is the main ingredient in prepared mustard. Mustard flour is a great way to add heat to a dry rub without the lingering intensity that you get from chili peppers. Mustard flour gives a mild heat to food. It is as delicious in salad dressings as it is on hot dogs and in dry rubs. It is a great addition to vinaigrettes.