Step 1: Make the Chocolate Cookies
• Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
• In a bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
• In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
• Beat in the egg and vanilla extract until well combined.
• Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

Step 2: Bake the Cookies
• Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
• Bake for 10–12 minutes, or until the edges are set. The centers may look slightly soft; they will firm up as they cool.
• Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

Step 3: Prepare the Peppermint Filling
• In a mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar and mix until combined.
• Stir in the peppermint extract and add enough milk to achieve a smooth, spreadable consistency.

Step 4: Assemble the Sandwich Cookies
• Once the cookies are completely cooled, spread a generous amount of peppermint filling on the flat side of half of the cookies. Top with the remaining cookies, pressing gently to create a sandwich.
• If desired, roll the edges of the filled cookies in crushed peppermint candies for a festive touch.