In a medium size bowl, whisk together the flour, cocoa, baking soda, and salt. Set aside.
In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on medium-high speed until light and fluffy (approximately 2-3 minutes). Turn the mixer speed down to medium, add the egg and vanilla, and beat again until combined.
With the mixer on low, slowly add the flour/cocoa mixture and beat until just combined. Add the chopped semi-sweet chocolate and continue mixing until evenly dispersed. Cover dough and refrigerate for at least 1 hour and up to 3 days.
When you are ready to bake the cookies, preheat oven to 350ºF (177ºC). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
Using a cookie scoop (I use this #50 cookie scoop for all of my standard size cookies), roll the dough into balls and place on prepared baking sheet. Bake for 8-10 minutes, or until edges are set and center is no longer shiny. Remove from oven and allow to cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.
PEPPERMINT WHITE CHOCOLATE FROSTING
Melt the white chocolate in a double boiler, on the stove, or in the microwave2. My preferred method is a small saucepan on the stove. Set melted white chocolate aside to cool at room temperature for at least 20 minutes.
In a medium size bowl with a handheld mixer or in the bowl of a stand mixer fitted with a paddle attachment, beat the butter on medium speed for 1 minute. Reduce the mixer speed to low and slowly add the powdered sugar and allow it to start to come together.
Stir the cooled white chocolate so that it is smooth, and then add it to the butter/sugar mixture. Increase the mixer to medium speed and beat for 2 minutes until creamy and well-combined.
Reduce the mixer speed one more time to low, then add the vanilla and peppermint extracts and salt. Slowly add the cream and stop when you have reached the consistency you prefer. Add the chopped white chocolate and stir or blend again until evenly dispersed.
Use a small offset spatula to spread frosting onto each cookie. Garnish with crushed candy canes. Cookies stay fresh covered at room temperature up to 3 days. Baked cookies can be frozen, up to 2 months. Rolled cookie dough can be frozen, up to 3 months. Place on baking sheet and add 1-2 minutes to baking time.
Notes
White chocolate: you can use bar chocolate (chopped) or white chocolate chips (do not chop). Your frosting with be the smoothest with bar chocolate.
Melt the white chocolate in the microwave: place chopped white chocolate in a microwave-safe bowl (I usually use a glass liquid measuring cup), and microwave in 20-second increments, stirring after every 20 seconds until completely smooth.
Candy canes: while I like to use candy canes for the topping, you could use pre-crushed peppermint candy canes, crushed starlight mints, or any kind of chocolate and/or peppermint topping you prefer. Candy canes or crushed peppermints will lend the most authentic taste and feel to traditional peppermint bark we're going for.
All nutritional values are approximate and provided to the reader as a courtesy. Changing ingredients and/or quantities will alter the estimated nutritional calculations.