Preheat oven to 375° F. Line cookie sheet with parchment paper.
In a mixing bowl, combine butter and sugars until well mixed.
Add eggs and vanilla, and stir until combined.
Add flour, cocoa powder, cornstarch, baking powder, and salt and stir until combined.
Stir in the peppermint chips.
Form balls of dough, about 1-2 tablespoons and place on lined baking sheet.
Bake for 10 minutes. Cool on the baking sheet for approximately 5 minutes, then transfer cookies to wire rack to cool completely.
Notes
The salt measurement is based on using Diamond Crystal Kosher Salt. Other salts will have a saltier taste. If using another type of salt such as sea salt, or especially table salt, the measurement will need to be reduced.For a fun twist, add 1-2 tablespoons of espresso powder to create a peppermint mocha cookie.When stirring in the dry ingredients, mix just until streaky as you still continue to mix when the peppermint chips are added. This will help to prevent over-mixing.