Place the chocolate bark in a double boiler and melt. When melted add 10 - 12 drops of the Peppermint oil and stir well.
Drop the Ritz crackers in the chocolate one at a time, coating completely. Use a fork to flip the cracker to make sure it is coated on both sides. Then lift the chocolate covered cracker out of the chocolate using the fork. You may want to gently tap the side of the pot to gently remove excess chocolate.
Place the "cookie" on waxed paper to cool and let the chocolate set. Once cooled place in an airtight container to store.
One package of chocolate will make almost the entire box of crackers. I have even let this cool if I didn't have time to use all of the chocolate and remelted it and continued later with great results.
The key to these is using peppermint oil. It will not set up if using peppermint extract. I have made these for years and the peppermint oil will last for years.