Preheat the oven to 350F. Add 1/2 cup (one stick) of butter to a pan over medium heat. Stir as the butter melts and continue to stir as the butter begins to bubble. After a few minutes the butter will start to foam up. When the foam subsides, you'll be left with brown, nutty smelling butter.
Carefully pour the brown butter into a bowl. Add the remaining 1/4 cup of butter to the bowl and stir. The cold butter will melt into the brown butter and help cool it down.
Add the butter mixture to a bowl. Add brown sugar and white sugar and use a stand mixer or hand mixer to combine until lighter and fluffy, about 3-4 minutes.
Add the egg + egg yolk and vanilla and mix until fully incorporated.
Equally divide the batter into two bowls. I recommend using a kitchen scale to be exact, but if you don't have a kitchen scale, use measuring cups or just eyeball it!
To one half of the dough, add the remaining dry ingredients for the plain dough: flour, baking soda, and salt. Mix until just combined. Add the semi-sweet chocolate chips and peppermint chunks, and mix until just combined.
To the other half of the dough, add the remaining dry ingredients for the chocolate dough: flour, cocoa powder, baking soda, and salt. Mix until just combined. Add the white chocolate chips and peppermint chunks, and mix until just combined.
Using a large cookie scoop (about 1/4 cup), scoop about 1 tbsp of the plain dough, then 1 tbsp of the chocolate dough, the repeat until the scoop is full. Place the dough ball on a parchment lined baking sheet, then repeat the process with the remaining dough.
Place up to 4 large dough balls on a baking sheet and bake for 8-9 minutes. Let them cool for 10 minutes, then transfer to a cooling rack. Enjoy!
