Mud cake is a dense, fudgy, chocolate-heavy cake with a texture closer to firm pudding or truffle than to a typical sponge cake; the name refers to that thick, muddy consistency.
It is made mostly from dark chocolate, butter, eggs, and a small amount of flour, with chocolate often making up half or more of the recipe by weight; the low flour and high fat content give it its dense, moist crumb rather than the airy structure of a classic butter or sponge cake. It is usually baked in a springform pan at a low temperature, around 325°F (165°C), for 45 minutes to just over an hour.
Mud cake is most strongly associated with Australian bakeries and celebration cakes today, though the style descends from the dense, fudgy cakes of the American South, popularly called Mississippi mud cake or mud pie. It should not be confused with a flourless chocolate cake, which uses no flour at all and relies on whipped eggs rather than a small amount of flour for structure.