Step 1: Prepare the Cookie Dough

1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
3. In a large bowl, beat together the softened butter, granulated sugar, and brown sugar until light and fluffy.
4. Add the egg, vanilla extract, and peppermint extract to the butter mixture. Mix until well combined.
5. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Step 2: Bake the Cookies

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

Step 3: Prepare the Mint Filling

1. In a medium bowl, beat the softened butter until creamy.
2. Gradually add the powdered sugar and heavy cream, mixing until smooth and fluffy.
3. Stir in the peppermint extract and a few drops of green food coloring if desired, mixing until well combined.

Step 4: Assemble the Sandwich Cookies

1. Once the cookies are completely cool, spread a generous amount of mint filling on the bottom of half of the cookies.
2. Top with the remaining cookies, pressing gently to create a sandwich.

Serve

1. Serve immediately or store in an airtight container. These cookies are best enjoyed within a few days.