Preheat the oven to 350℉ (175℃). Line two baking sheets with parchment paper.
Add softened, unsalted butter, light brown sugar, and granulated sugar to a large bowl. Cream the butter and sugars with an electric mixer for two minutes, or until light and fluffy.
To the creamed butter and sugars add the egg yolks, peppermint extract and vanilla. Mix on medium speed for two minutes, or until the mixture is pale in color and fluffy.
Add in the flour, cocoa powder, baking powder, baking soda and salt and mix until combined. (Please see recipe notes on properly measuring the flour for these cookies. Too much flour will ruin the cookies consistency.)
Add in the peppermint chips and chocolate chips and mix just until incorporated.
Scoop the dough into 18 balls and arrange them two inches apart on the prepared baking sheets.
Bake the cookies for 10-12 minutes. (10 for chewier centers, 12 for crispier edges.)
After baking, let the cookies cool on the baking sheets for two minutes, then transfer them to a cooling rack to cool for 10 minutes. Sprinkle crushed candy canes over the cookies if you wish and then serve!
Store left overs in an airtight container for up to three days.