I use the baby food freezing molds because I have them handy but I have it on good authority that regular ice cube trays work perfectly. (foodlets.com)
Regular ice cube trays aren't designed for the storage of breast milk, so they shouldn't be used for this purpose. (verywellfamily.com)
(As a side note, freezing coffee in ice cube trays is also a great way to keep (unfrozen) cups of iced coffee cold without watering them down with regular ice cubes.) (pumpsandiron.com)