Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Whisk together the flour, cocoa powder, baking soda, and salt in a bowl.

Cream the softened butter, granulated sugar, and brown sugar together until light and fluffy, about 2-3 minutes.

Add the egg, vanilla extract, and peppermint extract to the butter mixture. Mix until well combined.

Combine the dry ingredients with the wet mixture, stirring until just incorporated.

Scoop rounded tablespoons of dough and drop them onto your prepared baking sheet, spacing them about 2 inches apart.

Bake in your preheated oven for 10-12 minutes, or until the edges are set.

Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.

Beat the softened butter in a medium bowl until creamy.

Gradually add the powdered sugar and heavy cream, mixing until you achieve a smooth consistency.

Stir in the peppermint extract and a few drops of green food coloring if desired.

Spread a generous amount of mint filling onto the bottom of half of the cookies once they are completely cool.

Assemble by topping with the remaining cookies, pressing gently to form a sandwich.

Serve immediately or store in an airtight container for later enjoyment.