• 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.

Buttercream Frosting

• In a large bowl with an electric mixer whip your butter. Add in the powdered sugar, vanilla extract, and peppermint extract and whip again. Add in the milk and vanilla and beat again until light and fluffy. 
• Transfer the buttercream to a Ziploc bag with the tip snipped off or a piping bag with a round tip. Pipe the buttercream in a swirl on top of the cookies. Break the bark into pieces and place them on top of the cookies.