Frozen are perfect. Your corys will go absolutely nuts for them too
Take a couple, put them in a dish in some water and microwave them for about 20-30 seconds. Either wait for them to cool, or drain them and run cold water over them (which is what I do because I'm impatient). Once cool, pop them out of their shells, crush them between your fingers as much as you can (so the pieces are as small as possible), and drop them in the tank. I'd only do a couple at first until your fish get the idea that it's food
:EDIT: Sadguppy beat me to it
:EDIT: Sadguppy beat me to it