Thanks to the jester for cluing me to peppermint oil.
I re-made my thin mints with peppermint oil instead of extract, and they were much much better!
I went to GNC and got a small bottle of their NOW brand peppermint oil for $5. For about 8 ounces of chocolate I used three good shakes of the bottle (there's a pour controller built into it, but it's not an eyedropper. It's what I call a "shake and squirt" thing).
Another great thing I did was put about two drops in a quart of water. Very refreshing, and it did wonders for a touch of upset stomach I was experiencing. If you've ever had Mint water and liked it, save yourself some money and buy peppermint oil instead. So refreshing. When the weather gets warm I plan to take this on jogs and even to use a stronger version to soak hand towels for wiping down on a hot day.