In a medium bowl, sift together the flour, cocoa powder, salt and baking soda. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together the softened butter, granulated sugar and brown sugar. Beat on medium speed until light and fluffy (about 2-3 minutes).
Add the eggs, egg yolk and vanilla extract. Beat until well blended, scraping down the sides of the bowl as necessary.
Turn the mixer to low. Gradually add the flour mixture to the butter mixture. Mix until just combined and no dry streaks remain. Fold in peppermint and chocolate chips with mixer on low or by hand.
Drop by rounded teaspoons onto parchment-lined or lightly greased cookie sheet. Bake at 350°F for 8-10 minutes. The edges should look set/crinkly but the center will still look soft. Remove from oven and let cool on pan for 10 minutes before transferring to a wire baking rack to continue cooling.
Notes
Peppermint Chips - I like to use coarsely chopped Andes Peppermint Crunch Candy. Cookies pictured use Ghirardelli Peppermint Baking Chips. Recipe tested with Hershey’s Cocoa Powder, Hershey’s Special Dark Chocolate Chips.