In a large mixing bowl, combine softened unsalted butter, confectioners' sugar, vanilla extract, table salt, and peppermint oil. Using an electric mixer, beat the mixture on medium speed for about 30 seconds until combined.

Gradually add all-purpose flour and cocoa powder to the butter mixture, mixing on low speed until a smooth and cohesive dough forms.

Gently fold in the bittersweet chocolate wafers, white chocolate, and crushed hard peppermint candies using a spatula.

Using parchment paper, transfer the dough and shape it into a log approximately 1.5 inches wide and 11 inches long. Wrap the log tightly in the parchment and refrigerate it for a minimum of 2 hours.

Preheat your oven to 350°F (175°C). Prepare your baking sheets by lining them with parchment paper or a silicone baking mat.

Unwrap the chilled dough log and slice it into ½-inch thick rounds using a sharp knife. Place the slices on the prepared baking sheets.

Bake the cookies in the preheated oven for 12 to 14 minutes, rotating the pans halfway through for even baking.

Allow the cookies to cool on the baking sheets for about 5 minutes before transferring to wire racks to cool completely.