Preheat oven to 350°. Line a baking sheet with parchment paper or silicone mat, set aside.
In a medium bowl, whisk together flour, cornstarch, baking powder, and salt. Set aside.
In a large metal bowl over a pot of simmering water (double boiler), melt bittersweet chocolate, 1 cup of the semisweet chocolate chips, and butter. Stir often until melted, combined well and smooth. Remove from heat and let cool for about 5 minutes.
In a stand mixer, add eggs, brown sugar and vanilla. Mix on low until combined well. Gradually add cooled chocolate mixture. Next add flour mixture and scrape sides if needed. Fold in chocolate chips and crushed candy canes. (You can also mix all of this by hand)
Using a 1.5 inch cookie scoop, place dough on cookie sheet leaving 2 inches between each one. (They will spread while baking but as the dough sets longer between batches, it gets firmer with less spreading.)
Bake for 9-11 minutes or until edges are just set and centers slightly gooey. Remove and let sit on baking sheet for a few minutes before transferring to cooling rack.