A mudcake is an action while during intercourse the male proceeds to drop a steaming hot dump on the females stomach and then ejaculates onto it to imitate putting icing on a cake.
Person 1: Dude I made a fat MudCake on my girls back yesterday
you and your lover have almost finished some hardcore sex, so you pull out, take a shit on their chest stomach then ejaculate on the shit like you were putting mustard on a hotdog, then you or your lover eat the faeces.