Creating these Brownie Cookie Dough Sandwich Cookies requires a bit of attention, but the results are well worth it. Follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.

2. Prepare the Brownie Mixture:
– In a large mixing bowl, whisk together the melted butter and granulated sugar until smooth.
– Add the eggs, one at a time, stirring well after each addition. Mix in the vanilla extract.
– In a separate bowl, combine the flour, cocoa powder, baking powder, and salt.
– Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

3. Bake the Brownie Cookies:
– Drop tablespoon-sized balls of brownie batter onto the prepared baking sheets, spacing them about 2 inches apart.
– Bake for 12-15 minutes. The edges should be set, and the middle may look slightly underbaked.
– Remove from the oven and allow to cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely.

4. Prepare the Cookie Dough Filling:
– In a medium bowl, cream together the softened butter and brown sugar until fluffy.
– Mix in the milk and vanilla until well combined.
– Gradually add in the all-purpose flour, stirring until the mixture is soft.
– Fold in semi-sweet chocolate chips until evenly distributed.

5. Assemble the Cookies:
– Once the brownie cookies are completely cool, spread about a tablespoon of cookie dough filling onto the flat side of half the cookies.
– Top with another brownie cookie, flat side down, creating a sandwich. Press gently to combine.

6. Chill and Set:
– Place the completed sandwiches in the refrigerator for at least 30 minutes to allow the filling to firm up.

By following these steps closely, you will have created rich and delectable Brownie Cookie Dough Sandwich Cookies that everyone will love.