We exposed an old brick chimney. Had a similar layer of very hard plaster - looked like concrete. I chipped it away by hand, mainly with the claw side of an Estwing framing hammer, but used a cold chisel here and there as needed. It was a lot of messy work, but 95% of the plaster did come off and left the brick in decent condition. It looks great now, although I'm not sure I would have done it if I'd known how big a job it would be.