Put the dark chocolate in a microwave safe bowl and microwave for 1 minute. Take it out and stir. Put it back in for 30 seconds, take it out again and stir. If the chocolate is not completely melted and pourable, put it back in for 15 second intervals until it is. I use a silicone spatula for the stirring. 2 minutes should be the maximum you need, unless you have a really old microwave.
Pour the chocolate out onto a parchment lined baking sheet. Spread it evenly into approximately an 11x8 rectangle. Set aside to cool a bit and start to firm up.
When the dark chocolate is firm, melt the white chocolate the same way. Be careful, white chocolate is temperamental and reacts badly to too much heat.
Pour the white chocolate over the dark, and, working quickly, spread it out evenly over the top.
Sprinkle the crushed candy canes evenly over the hot white chocolate, pushing down slightly so they will adhere once the chocolate is cooled and cut.
Set aside to firm up. When it is firm, cut or break into pieces. Some of the pieces may fall apart a little bit, don't worry, no one will care. If you want more even cuts, cut it while at room temperature, and use a sharp knife dipped in very hot water (and then dried) for each cut. Otherwise, hack away at it. The taste will be the same either way!