Coffee cake is cake intended to be eaten with, or flavored with, coffee. British coffee cake is a sponge flavoured with coffee. They are generally round and consist of two layers separated by coffee flavoured butter icing, which also covers the top of the cake. Walnuts are a common addition to coffee cakes. In the United States, coffee cake generally refers to a sweet cake intended to be eaten with coffee or tea (like tea cake).
Coffee cakes, as an accompaniment for coffee, are often single layer, flavored with either fruit or cinnamon, and leavened with either baking soda (or baking powder), which results in a more cake-like texture, or yeast, which results in a more bread-like texture. Sour cream is used in traditional American coffee cakes to both impart a tart flavor and activate baking soda used as a leavening agent.