The recipe calls for a whole box of raisin bran. Once the batter is mixed, it needs to sit overnight in the refrigerator. The buttermilk and baking soda cause the batter to bubble. (I don't keep a sourdough starter, so this is the most bubbly thing I make!)
I like to make this recipe and freeze some of them, so that when we run out of milk for cereal some morning I can just reach into the freezer and produce muffins!