Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a large mixing bowl, combine the cocoa powder, baking soda, salt, and flour. Whisk together until well combined.
Step 3: In another bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Step 4: Add the egg and vanilla to the butter-sugar mixture, beating until fully incorporated.
Step 5: Gradually add the dry mixture to the wet mixture, mixing until just combined.
Step 6: Use a cookie scoop to drop rounded balls of dough onto a parchment-lined baking sheet, spacing them about 2 inches apart. Flatten slightly.
Step 7: Bake for 10-12 minutes until edges are set. They will firm up as they cool.
Step 8: Let cookies cool on the baking sheet for 5 minutes before transferring to a cooling rack.
Step 9: In a mixing bowl, beat the unsalted butter and salt until creamy. Gradually add the powdered sugar and mix until smooth. Stir in peppermint extract and cream until fluffy.
Step 10: Fold in mini chocolate chips into the mint filling.
Step 11: Spread mint filling onto the flat side of one cookie, top with another cookie to create a sandwich.
Step 12: Enjoy your Homemade Mint Chip Oreos fresh or share with friends!