Preheat the oven to 350 degrees Fahrenheit and line a baking sheet with parchment paper or a Silpat mat. Set baking sheet aside.: The warm dry heat will cause the edges to set while the center remains tender, and lining the sheet prevents sticking and promotes even browning. You should feel the kitchen shift as the oven reaches temperature, and the air will carry a faint warm aroma. A common mistake is not letting the oven fully preheat which results in uneven rise and underbaked centers.
Sift the flour, cocoa powder, baking soda, baking powder, and salt in a medium-sized bowl. Set aside.: Sifting aerates and removes clumps, especially in the cocoa powder , resulting in a smooth, even batter. Rub a little of the mixture between your fingers to check for lumps, and watch the powder fall light and even. If you skip sifting, you risk pockets of dry cocoa that create streaks in the dough.
In a large mixing bowl, beat the butter, brown sugar, and granulated sugar on medium-high speed using an electric mixer. Beat for 2 minutes, until light and creamy.: The creaming incorporates air into the mixture, which contributes to a pillowy interior. You should see the mixture lighten in color and become fluffy, and you can hear the mixer change timbre as the texture changes. Overbeating can cause the butter to soften too much and yield excess spread, so stop when it looks pale and aerated.
Scrape the sides of the bowl, then add the egg, vanilla extract, and peppermint extract. Beat for an additional minute to completely combine.: Scraping ensures that all the creamed butter and sugars are reincorporated, giving even flavor. The batter will smell sweet and minty, and emulsion looks smooth and glossy. If you skip scraping, you may end up with streaks of uneven texture in the cookies.
Add the dry ingredients to the mixing bowl with wet ingredients. Mix on low speed until just combined. Then stir in the chocolate chips. The batter will be thick.: Mixing on low prevents overworking the gluten in the flour , which keeps the cookies tender. You want to stop when you see no streaks of dry flour, and then fold in the chocolate chips until evenly distributed. Overmixing leads to tough cookies, while undermixing leaves dry pockets.
Use a large cookie scoop to portion into 3 tablespoon-sized balls.: Portioning consistently ensures even baking so pieces finish at the same time. The dough should feel dense and slightly tacky, holding its shape when scooped. If the dough is too sticky, chill briefly; if too dry, check measurements. Uneven scoops produce varying textures and baking times.
Place cookie dough balls on a parchment-lined baking sheet at least 2 inches apart and bake for 7-9 minutes, until the top of the cookie has a dry appearance.: Space prevents spreading together and allows heat to circulate, producing uniform edges. Watch for the tops to lose their glossy wet look and for the edges to set but not brown heavily. You may hear faint crackling as steam escapes. Leaving them too long will dry the centers, while taking them out too early yields underdeveloped structure.
Remove from the oven. Immediately sprinkle crushed peppermints over the cookies. You can also press in additional chocolate chips, if desired.: The residual heat helps the crushed peppermint candy adhere and partially melt into the surface, creating glossy, jewel like patches. Pressing in extra chocolate chips gives visual appeal and melty pockets. A common error is waiting too long to add the candy, which prevents adhesion and can cause pieces to fall off.
Let the cookies cool on the cookie sheet for 5 minutes. Then transfer to a cooling rack and cool completely.: The cookies finish setting as they cool, and five minutes allows them to firm enough to move without tearing. You should feel the base cool slightly and notice the cookies relax into their final shape. If you try to move them too soon, they may break or deform; if you leave them too long on a hot sheet, the bottoms can overbrown.