We embed a mouse brain (unfixed) in OCT, inside a Tissue tek disposable mold
and snap freeze it with a dry ice/isopentane slurry, and section at  -16C.
When you add isopentane to dry ice, it bubbles violently, when that stops
the correct temp has been reached for snap freezing of tissue.
 
You can also cool hexane with dry ice (put hexane in beaker, surround beaker
with  dry ice until hexane is cold (give it some time) and freeze with that,
a bit slower, but some prefer a tidge slower for brain, with excellent results

or try the dry ice slurry with hexane instead of isopentane.

Good luck
Gayle Callis