Melt dark chocolate in a double boiler or a microwave. Once melted, immediately stir in ¼ teaspoon of peppermint extract.
Once the chocolate is melted, spread into an even layer on the sheet pan (make sure to use parchment paper or the chocolate will stick). Place the tray in the refrigerator for 10-15 minutes, until just set.
Repeat the melting process with the white chocolate, then stir in remaining peppermint extract. Once the semisweet chocolate layer is mostly set, spread the white chocolate over the top in an even layer.
Immediately sprinkle crushed candy canes evenly over the top of the white chocolate. Place the bark in the refrigerator for 1 hour (or more) to fully set. Break into pieces with a knife or your hands.