Prepare the Cookie Base:
Preheat the oven to 350°F and line a baking sheet with parchment paper or a silicone baking mat.
In a small bowl, whisk together the flour, cocoa powder, cornstarch, baking powder, and salt. Set aside.
½ cup all-purpose flour, 3 tablespoon cocoa powder, ½ tablespoon cornstarch, ½ teaspoon baking powder, ¼ teaspoon salt
In a microwave-safe bowl, melt the butter and dark chocolate chips in 30-second intervals, stirring between each, until smooth. Mix in the cocoa powder until combined and set aside.
¼ cup unsalted butter, ¾ cup dark chocolate chips or chunks
Using a hand mixer or stand mixer, whisk the egg, brown sugar, and granulated sugar on medium-high speed for about 5 minutes, or until light, fluffy, and doubled in volume. Add the vanilla extract and mix.
1 large egg, ¼ cup brown sugar, ¼ cup granulated sugar, ½ teaspoon vanilla extract
Gently fold the melted chocolate mixture into the egg mixture, followed by the dry ingredients. Mix just until combined.
Scoop the dough into 2-tablespoon-sized balls and place them on the baking sheet about 2 inches apart.
Bake for 9 minutes, then let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.
Prepare the Peppermint Bark Topping:
Melt the white chocolate chips in a microwave-safe bowl in 20-second intervals, stirring between each, until smooth.
10 oz white chocolate chips
Stir in the peppermint extract and oil to thin out the white chocolate slightly for easy spreading.
1/4 teaspoon peppermint extract, 1 teaspoon neutral oil
Once the cookies are completely cooled, dip the cookies into the melted white chocolate to coat them well and place them back onto the baking sheet.
Sprinkle crushed candy canes over the white chocolate before it sets.
¼ cup crushed candy canes
Let the cookies sit at room temperature or in the refrigerator until the white chocolate is firm.