Serious Eats - seriouseats.com

If you can get second-distillation peppermint oil, it has a more candy cane/thin mint kind of flavor, and a higher menthol content, which gives it that nice "cool" flavor as well. I buy mine online, from a soap making supply company, of all places.

As for your butter/shortening issue, butter always tastes better. I use clarified butter when making chocolate glaze, which removes the extra water problem. I dip meringues, chill them for 5 minutes, and bring them back out and they sit just fine at room temperature.

I use 8 oz melted dark chocolate, with 2 oz clarified butter. You'd probably only need 2 to 4 drops of peppermint oil for that much chocolate.