1. In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.

2. In a large mixing bowl, cream butter, brown sugar, and granulated sugar until light and fluffy (about 2-3 minutes).

3. Add eggs one at a time, beating well after each addition. Mix in vanilla extract.

4. Gradually add dry ingredients to wet ingredients, mixing on low speed until a soft dough forms. Do not overmix.

5. Cover dough and chill for at least 1 hour.

6. Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

7. Scoop 1-tablespoon portions of dough, roll into balls, flatten slightly, and place 2 inches apart on the baking sheet.

8. Bake for 8-10 minutes until edges are set. Centers will still look soft.

9. Cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

10. Make the buttercream by beating softened butter until smooth. Add powdered sugar gradually, followed by vanilla and heavy cream. Beat until fluffy.

11. Spread or pipe buttercream onto the flat side of half the cookies. Top with remaining cookies and press gently.

12. Optional: roll edges in sprinkles or chocolate shavings.