• Mix dry ingredients. Whisk the flour, cocoa powder and salt together in a bowl. Set aside.
• Cream. Add the butter, sugar and vanilla extract into a medium bowl and beat with an electric beater, until smooth and creamy (1 to 2 minutes). Add the egg white and beat for about 30 seconds.
• Finish dough. Add the dry ingredients and mix at low speed for 30 seconds. The dough should be slightly firm, but not overly stiff.
• Roll and chill. Roll the dough between two sheets of parchment paper until it is slightly less than ¼ inch thick. Transfer the dough, still between the parchment, onto a large baking sheet and refrigerate for at least 4 hours (or overnight) until firm.
• Prep. Preheat oven to 350°F (177°C). Line a large baking sheet with parchment paper.
• Cut into squares. Cut the dough into 2-inch squares or use your favorite cookie cutter to create any shape you like. Transfer the cut cookies to the baking sheet, spacing them about 1 inch apart, as they won’t spread much during baking. If the cookie dough has softened during this time, then chill it briefly before baking.

  👉 Gather the dough scraps, re-roll them, chill again until firm, and cut out more cookies.
• Bake. Bake for 15–20 minutes, rotating the pan halfway through. The cookies are done when they feel slightly firm in the center. Place the pan on a wire rack to cool completely.

Decorate

• Melt chocolate. Melt the chocolate chips and oil in the microwave, stirring until smooth and fully melted.
• Decorate. Spread melted chocolate over about half of each cookie, then place the cookies on parchment paper. While the chocolate is still warm, sprinkle with crushed candy canes. Let the cookies sit at room temperature for a few hours until the chocolate sets, or place them in the fridge to set more quickly.