Heat oven to 350°F. Line two baking sheets with parchment paper.
Whisk the flour, cocoa powder, baking soda and salt in a medium bowl.
Beat the butter, brown sugar and granulated sugar in a large bowl until light, about 2 minutes. Beat in the egg and vanilla.
Add the dry ingredients and mix on low just until combined. Stir in the chocolate chips and 1/4 cup of the crushed candy canes.
Scoop rounded tablespoons of dough 2 inches apart onto the prepared sheets. Press the remaining crushed candy canes onto the tops.
Bake 10-12 minutes, until the edges are set and the centers still look slightly underdone.
Cool on the sheets 5 minutes, then transfer to a wire rack to cool completely.