From Homestar Runner Wiki
Grumblecakes are "like a little cake with some, like, steam and sugar". Only liars and thieves eat them, and, according to The Cheat, those people go to prison. They nevertheless seem pretty popular, at least with Homestar and Strong Bad.
They first appeared in the email stupid stuff, when Strong Bad made a bet that if he could get Homestar to say something smart, Kevin Grumbles would have to make him some of his famous grumblecakes. Unfortunately for him, the nefarious Kevin Grumbles had also signed a deal with Homestar, thereby tricking Strong Bad out of the cakes and making him look stupid as well.
Conversely, The Cheatcakes are a popular brand of confectionery made by The Cheat, yet for some reason Strong Bad does not seem to like them as much.