This expression is used for everything from food (like these English muffins) to how to clean surfaces. It's use originated in the 14th century with 'nooks and crannies' meaning small 'out-of-the-way' locations in an object or a room, with 'nook' meaning a distant corner and 'cranny' meaning a crack or gap. For this mug bread, it simply refers to those beautiful gaps with toasty edges that get filled with luxurious, melted butter.