A mud cake is a cake that is so dense, dark and chocolatey that it supposedly resembles mud. Whilst I don’t quite think that’s true, it is really one of the most luxurious and rich chocolate cakes rather than being bouncy and airy. A regular sponge cake is usually baked at a higher temperature like 180-190 degrees and much quicker which allows for a bigger rise and an airier crumb whereas mud cakes are cooked at a lower temp for longer which gives it a tighter crumb, and a more dense, rich outcome.