In a mixing bowl whisk together flour, baking soda, baking powder and salt. Set aside.: You will first notice a light, powdery scent as you whisk the dry flour and leaveners together, and the mixture should look uniformly pale without any streaks of baking soda. This blending ensures even distribution so pockets of leavener do not create uneven rise, which is why I always whisk until the color is consistent. A common mistake is skimping on this step which can produce bits of salty or metallic taste where the baking soda concentrates, so take a few extra seconds to make it even.
In the bowl of an electric stand mixer fitted with the paddle attachment cream together butter, granulated sugar and brown sugar until well blended. Scrape down bowl.: As you cream the room temperature butter with both sugars, you will see the mixture turn lighter in color and become fluffy, which traps air and contributes to tenderness. The sound changes to a gentle whipping as the paddle incorporates the sugars, and you should scrape the bowl to ensure no pockets of butter remain. Overcreaming can produce excessively soft dough that spreads too much, so stop once the mixture is pale and smooth, rather than ultra aerated.
Mix in egg, then blend in egg yolk, peppermint extract and vanilla extract. Scrape down bowl.: When you add the egg and extra egg yolk , you will notice the mixture briefly separate then come back together into a glossy batter, signaling proper emulsification. The scent will shift as the vanilla extract and peppermint extract bloom, giving a sweet and minty aroma. If the mixture looks curdled after adding eggs, it usually comes back together with a bit more mixing, but avoid overmixing which can toughen the final cookie.
Add in flour mixture and mix just until combined.: At this stage you should hear only a few soft clacks from the paddle as the dry flour disappears into the wet mixture, and visually you want pockets of flour to vanish without a glossy, overworked dough forming. Mixing until just combined maintains a tender crumb by limiting gluten development. A typical error here is continuing to mix until the dough looks perfectly smooth, which can produce dense cookies, so stop as soon as strands of flour are incorporated.
Pour in white chocolate chips and crushed candy canes and mix to combined.: Adding the white chocolate chips and crushed peppermint candy canes changes the texture instantly, scattering pale chips and flecks of red throughout the dough. You will feel slightly more resistance in the bowl, and the sound becomes more muted as mix-ins spread. Mixing briefly preserves the chips intact and prevents them from melting into the dough, which keeps pockets of creamy white chocolate. Avoid prolonged mixing which can break the chips and create streaks of white throughout the dough.
Scoop dough out using a slightly heaped #40 cookie scoop (about 1.5 heaped Tbsp or 37 g). Shape into balls and transfer to a parchment lined tray.: As you scoop, the dough should be pliable and hold its shape when rounded, with visible chips and peppermint pieces on the surface. The dough feels cool to the touch because of the butter , and shaping into neat balls helps promote even baking. A common oversight is using too large a scoop which increases bake time and can lead to underbaked centers, so keep to the recommended size for uniform results.
Chill cookie dough balls 20 minutes. Meanwhile preheat oven to 350 degrees and line baking sheets with parchment paper.: Chilling firms the dough, which helps control spread during baking and concentrates flavors. You will notice the dough becomes firmer and easier to handle after chilling, and the peppermint scent slightly intensifies. Preheating the oven while chilling ensures the oven is ready immediately so cookies go in at the correct temperature. If you skip chilling, the cookies may spread too thin, so resist the temptation to rush this step.
Transfer 12 cookie dough balls to lined cookie sheet, spacing 2-inches apart. Bake in preheated until until nearly baked through (they should appear puffed and slightly under-baked in center), about 12 to 14 minutes.: While baking you will see the cookies puff and the edges gain a soft golden hue, while the centers remain slightly domed and glossy. The kitchen will be filled with sweet white chocolate and mint notes as the chips begin to soften. Pulling them when the centers look a touch underbaked ensures a soft, chewy interior after cooling. A common mistake is baking until fully set, which yields a drier cookie, so watch for that slightly underdone center.
Let cool on baking sheet 5 minutes before transferring to a wire rack to cool. If you'd like you can press a few white chocolate chips into each cookie while cookies are still warm.: During this short cooling period the cookies continue to set, and the white chocolate chips will soften and adhere for a polished look. You should notice some steam rising initially then dissipating as the cookies stabilize, and the bottoms will become firm enough to move without breaking. Moving them too soon can cause breakage, so allow the brief rest. Pressing chips on top while warm gives that inviting glossy finish.
Repeat baking with remaining cookie dough.: Keep an eye on the second batch as oven temperatures fluctuate slightly between loads. You will likely see consistent results if you rotate sheets and maintain the same spacing and timing. A frequent error is overcrowding the pan which alters airflow and leads to uneven baking, so keep batches to the recommended count.
Once cookies are cool store in an airtight container at room temperature up to 3 days.: Fully cooled cookies should feel slightly springy but not soft to the touch before storing, and they will retain flavor when sealed in an airtight container. You may notice the peppermint crispness soften over time, but the overall taste remains excellent for a few days. Avoid storing while warm which traps steam and makes cookies soggy, that is an easy but common slip.