So, I left the cookies in a turned-off oven to dry overnight. When I got up in the morning, they were still a bit chewy around the middle. Pooh. So I turned the oven to 200 for a half hour, then turned it back off and let them sit around while I ate breakfast and went to the gym. When I got back they were just about perfect! Not quite as shatteringly dry as one might get in, say, December, but pretty darn good.
So I went to whip up the ganache. I'd been frustrated in my efforts to find "pure peppermint oil" (it was not at any of the four stores -- including Penzey's and the fancy chocolate shop -- John and I went to), so in the end I used peppermint extract. I used the microwave to melt the chocolate (as I always do) and think I let my impatience get the better of me . . . the chocolate heated too much and got a bit grainy. But as the chocolate layer was so thin, and delicious, I don't think it mattered all that much in the end.
Boy, were these good! When I make them again I'll use a pastry bag and make a point to make them a bit smaller, but otherwise I wouldn't change a thing. The cookies peeled off the foil like a dream, stayed stuck together, and tasted just like mint chocolate chip ice cream. I went the rustic route, so mine wouldn't have won any beauty contests, but with a little more attention they could really be showstoppers. Certainly everyone at the shindig I took them to gobbled them up and raved.