In a microwaveable bowl, combine butter and unsweetened chocolate. Microwave on 50% power for 1 minute, stir, then microwave again in 30 second intervals until chocolate is mostly melted. Stir to melt the remaining chunks. Transfer to a large mixing bowl and stir in sugar, eggs and vanilla extract.
In a separate bowl, whisk together flour, cocoa, baking powder and salt. Sprinkle flour mixture over chocolate mixture and stir until well combined. Stir in white chocolate chunks. Refrigerate at least 2 hours, or overnight.
Preheat oven to 350 degrees. Line 2 large baking sheets with parchment paper. Drop dough by heaping tablespoonfuls 2 to 3 inches apart. Bake 7 minutes, remove from oven and sprinkle with crushed candy canes. Return to oven 2-4 minutes longer, until edges are crackled and centres are just set. Remove and cool 5 minutes on baking sheet before transferring to wire racks to cool completely.
Bakers tip: Whenever a cookie recipe calls for melted chocolate, it is very important to 1) chill the dough for at least 2 hours to prevent flat cookies, and 2) line the baking sheets with parchment paper or they will likely stick.