Line a baking sheet with parchment paper. Using a pencil, draw a 9" by 10" rectangle lengthwise in the middle of the parchment. Flip the paper over so you can see the rectangle, but the pencil won't rub off.

Place 3/4's of the semisweet and bittersweet chocolate in a medium glass bowl. Microwave on high for 30 seconds, then stir the chocolate. Microwave on high again for 30 seconds, then stir the chocolate. Continue microwaving at 30 second intervals and stirring until the chocolate is just melted. Stir in the remaining semisweet and bittersweet chocolate until smooth. Microwave for an additional 15 seconds on high if the chocolate isn't fully melting.

Pour the chocolate on to the prepared baking sheet, and gently smooth it out to fit the rectangle (doesn't have to be perfect). Cover the surface of the chocolate with even rows of pretzels (see graphic above), gently pressing them in.

Melt white chocolate using the same method above (30 seconds of cooking, then stir). The white chocolate will melt fast since it's a smaller amount. Stir in peppermint extract. Pour white chocolate into a small zip top bag, close the bag, then cut off a little bit of one of the bottom edges (an improvised piping bag). Drizzle the white chocolate back and forth over the pretzels in a criss-cross pattern. You will have leftover white chocolate (great in coffee!). Finish by sprinkling the surface of the bark evenly with crushed candy cane (you'll have leftover here too).

Let the bark sit at room temperature for at least 2 hours, so the chocolate can firm up. Using a sharp knife, carefully cut the bark into pieces. Don't use your hands or excess force here, because the pretzels will break free from the chocolate. Store in an airtight container at room temperature for up to 2 weeks.