Preheat the oven to 350°F. Line two baking sheets with parchment paper.

In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In another bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined. Fold in the chocolate chips.

Drop tablespoon-sized balls of dough onto the prepared baking sheets, spacing them about 2 inches apart. Bake for 10-12 minutes or until the edges are golden. Let cool completely.

For the brownie layer, melt the butter in a saucepan over low heat. Stir in the sugar, then remove from heat and let cool slightly. Whisk in the eggs and vanilla extract.

In a separate bowl, combine the flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined. Fold in the chocolate chips.

Pour the batter into a greased 9x13 inch baking pan and bake for 20-25 minutes. Let cool completely.

For the cookie dough filling, beat together the softened butter, brown sugar, and granulated sugar until creamy. Mix in the milk and vanilla extract.

Gradually add the flour and salt, mixing until combined. Fold in the mini chocolate chips.

To assemble, spread a layer of cookie dough filling on the flat side of one cookie, then top with a brownie square. Place another cookie on top, flat side down, to create a sandwich. Repeat with remaining cookies and brownie squares.