Line a baking sheet with parchment paper.

In a medium microwave-safe bowl, melt the dark or semisweet chocolate at 50% power for 1 minute. Stir, then continue to heat in 20 second increments, stirring between each interval until melted.

Spread the dark chocolate in a large rectangle on the parchment paper, then let it begin to set slightly while you melt the white chocolate.

Melt the white chocolate in the same way as the dark chocolate, by microwaving at 50% power for 1 minute, then stirring and continuing to melt with short 20 second bursts of heat in the microwave, stirring between each burst until melted.

When the white chocolate is melted, stir in the peppermint extract and 1/2 of the crushed candy canes.

Drizzle the white chocolate over the partially set dark chocolate, gently nudging it around to cover the surface (or swirl, for a marbled effect). Immediately sprinkle with the remaining crushed candy canes.

Let the peppermint bark set at room temperature for 30-90 minutes until hard, then cut apart with a sharp knife or break apart with your hands inside plastic gloves or a ziploc bag so you don't leave fingerprints on the peppermint bark.

Store at room temperature for up to 2 weeks.