Creating Irresistible Brownie Cookie Dough Sandwich Cookies is a delightful process. Follow these steps for best results:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. Mix Brownie Ingredients: In a large bowl, combine melted butter, granulated sugar, and brown sugar. Mix until smooth.
3. Add Eggs and Vanilla: Beat in the eggs one at a time, followed by the vanilla extract. Ensure everything is well incorporated.
4. Incorporate Dry Ingredients: In another bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add this mixture to the wet ingredients, stirring until just combined.
5. Scoop Brownie Batter: Using a cookie scoop or tablespoon, drop rounded balls of brownie batter onto the prepared baking sheets, spacing them about 2 inches apart.
6. Bake: Bake in the preheated oven for 12-15 minutes. The edges should be set, while the centers remain soft. Allow them to cool on the baking sheets for 10 minutes before transferring to wire racks to cool completely.
7. Make Cookie Dough Filling: In a separate bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
8. Add Milk and Vanilla: Mix in the milk and vanilla extract until fully combined.
9. Incorporate Flour and Chocolate Chips: Gradually add the flour and mix until the dough comes together. Fold in the chocolate chips.
10. Assemble Cookies: Once the brownie cookies are completely cooled, spread a generous amount of cookie dough filling onto the bottom of one brownie cookie. Top with another brownie cookie to create a sandwich.
11. Repeat: Continue this process until all brownie cookies are filled and sandwiched together.

These steps will guide you in creating the ultimate dessert that is both delicious and indulgent.