Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Use a hand mixer or a stand mixer to whisk the butter and sugar together until it is creamy.
Add the egg, egg yolk, vanilla extract, and peppermint extract into the butter mix and whisk until it is fluffy.
In another bowl mix the flour, cocoa powder, baking soda, and salt together.
Pour the butter and egg mix into the flour mix and whisk together until it is well combined.
Mix in the chocolate chips. Use a cookie scoop to portion out the dough onto the lined baking tray. Give each cookie plenty of space to expand. Bake in the oven for 10–15 minutes.
Remove the cookies from the oven and let cool on the sheet or on a wire rack. Melt the white chocolate. Decorate the cooled cookies with the white chocolate and the crushed peppermint candies.