Preheat the oven to 375 degrees. Line two baking sheets with parchment paper and set aside.
Whisk together the flour, cocoa powder, baking powder, cornstarch, salt, and baking soda in a medium bowl until combined then set aside.
In a large bowl, beat the butter and sugar until smooth and creamy.
Add the peppermint extract and eggs . Only add one egg at a time, beating until blended between each egg.
Next slowly add in the dry ingredients and mix until well combined.
Fold in the dark chocolate chips and peppermint bark and stir to combine.
Use a cookie dough scoop or tablespoon to scoop out dough. Roll each scoop into a ball and place on the prepared baking sheet. Lightly press down cookie dough with your fingers. Then top the cookie dough with additional chopped peppermint bark.
Bake the cookies one batch at a time in the center of preheated oven for 10 to 12 minutes. Cookies are done when the edges are firm but the center is still soft.
Allow the cookies to cool on the baking sheet for at least five minutes before transferring to a wire cooling rack to finish cooling before serving.