Lewis & Short

mustācĕus, i, m., and mustācĕum, i, n., a must-cake or laurel-cake, a kind of wedding-cake mixed with must and baked on bay-leaves: mustaceos sic facito, etc., Cato, R. R. 121; Juv. 6, 200.
Prov.: laureolam in mustaceo quaerere, to look for a laurel-wreath in a cake, i. e. for fame in trifles, Cic. Att. 5, 20, 4.