1. Preheat your oven to 350°F and line two baking sheets with parchment paper.
2. In a large mixing bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.
3. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
4. In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until combined.
5. Fold in the chocolate chips. The dough will be thick and slightly sticky.
6. Using a tablespoon, scoop out dough and roll into balls. Place them on the prepared baking sheets, leaving about 2 inches apart.
7. Bake for 10-12 minutes, or until the edges are set and the centers are still soft. Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
8. To make the chocolate fudge filling, heat the heavy cream in a small saucepan over medium heat until it begins to simmer.
9. Remove from heat and pour over the chopped dark chocolate. Let it sit for 2 minutes, then whisk until smooth and glossy.
10. Once the cookies are completely cool, spread a generous amount of the chocolate fudge filling on the flat side of one cookie and sandwich with another cookie.

One of my personal tips is to allow the filled sandwiches to sit for a few minutes before serving. This gives the filling time to set slightly, making them easier to handle and even more delicious.