Line a baking tray with wax paper and set aside. Place unwrapped candies in a zip-top bag. With a mallet or hammer, crush the candy and set it aside.
Place the dark chocolate chips in a microwave-safe bowl. Microwave for 90 seconds at 70 percent power. Stir and continue to microwave at 15 second intervals until the chips are almost melted. Stir the chips using residual heat to completely melt the chips. Stir in the peppermint extract. Scrape the chocolate into a disposable decorating bag, twist the top and snip 3/8-inch from the tip.
Pipe 1 1/2-inch circles of chocolate on the prepared baking tray. Place a pretzel on top.
While piping the circles, microwave the white chocolate chips as you did the dark chocolate chips, and transfer the chocolate into a disposable decorating bag. Snip 1/4-inch from the tip. Pipe back and forth on each pretzel. Immediately sprinkle with crushed candy.
Let the chocolate bark pretzels set at room temperature for two hours or in the refrigerator for one hour.