Preheat oven to 325 degrees
Prepare the brownie batter per the package instructions.
On a cookie sheet lined with parchment paper, spoon drops of brownie batter leaving space for spreading
Bake for 10 to 12 minutes or until the tops of the brownies start to crack.
Allow to cool on the cookie sheet.
While the brownies are cooling, beat the brown sugar and butter on high speed until creamy and fluffy.
Add in the powdered sugar and mix until smooth.
Add in the flour and the salt. Blend until well combined.
Add the milk and vanilla. Mix well.
Fold in 3/4 cup of mini chocolate chips.
Spread this mixture on half of your "brownie cookies" then top with another brownie cookie.
For even more chocolatey goodness, you can press or roll some extra mini chocolate chips to the sides of the cookies.