I've had a chisel and a block/cutting board in one version or another for at least the last decade...
...and lately it's become one of my favorite notions because it makes opening buttonholes so easy. No cutting the edges or ends of the buttonhole threads...don't have to pull out the scissors and pins to make sure you don't chop up your buttonholes. No the chisel fits neatly inside of the buttonhole and cuts cleanly. I love it.
You can get your own version at Nancy's Notions for $6.99. It's a great deal and if you don't have one, you should add it to your notion collection.