Preheat your oven to 350 degrees. Line a baking sheet with parchment paper then set aside.
In a large bowl beat the butter, brown sugar, and white sugar together until light and fluffy. Next mix in the egg and vanilla extract until combined. Next mix in the flour, baking soda, cocoa powder, and salt and mix until a dough forms. Chill for 30 minutes.
Separate the dough into 6 balls and place them on the baking sheet. Bake the cookies for 10-12 minutes. Let the cookies cool on the baking sheet for 10 minutes then transfer them to a cooling rack. Cool the cookies completely. Meanwhile, make your peppermint bark.
Peppermint Bark
In a microwave-safe bowl melt your semisweet chocolate chips in 30-second intervals until melted and smooth. Repeat with the white chocolate. Spread the melted semisweet chocolate out thin on a piece of parchment paper. Spread the white chocolate on top of that. Sprinkle with the crushed peppermint and set aside to set.