1. Preheat your oven to 350°F and line two baking sheets with parchment paper.
2. In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy, about 3 minutes.
3. Add in the vanilla extract and eggs, one at a time, mixing well after each addition.
4. In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
5. Gradually add the dry ingredients to the wet mixture, mixing until just combined.
6. Fold in mini chocolate chips, then scoop tablespoon-sized mounds of dough onto the prepared baking sheets, spacing them about 2 inches apart.
7. Bake for 10-12 minutes, or until the edges are set and the centers look slightly underbaked. This ensures a fudgy texture.
8. Allow cookies to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.
9. For the cookie dough filling, in a mixing bowl, combine butter, sweetened condensed milk, and vanilla extract until smooth.
10. Gradually mix in flour and fold in the remaining mini chocolate chips.
11. Once cookies are cooled, spread a generous amount of cookie dough filling on the flat side of one brownie cookie, then sandwich with another.

As you assemble these delightful sandwiches, remember the joy of creating something from scratch. It’s a tradition that has been passed down through my family, and I hope it becomes a cherished ritual in yours as well.