Preheat the oven to 350°F (175°C) and prepare baking sheets.
In a mixing bowl, cream the unsalted butter and granulated sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Fold in the mini chocolate chips.
Drop tablespoon-sized scoops of dough onto prepared baking sheets, spacing them about 2 inches apart.
Bake in the preheated oven for 10-12 minutes, until the edges are set but the centers are still soft.
Remove from the oven and let the cookies cool on baking sheets for 5 minutes before transferring to wire racks.
For the frosting, cream the unsalted butter and brown sugar until smooth.
Add the flour and vanilla extract, then mix until combined.
Add milk, one tablespoon at a time, until the desired consistency is reached.
Spread frosting on the flat side of half of the brownie cookies, then top with the remaining cookies.
Serve the sandwich brownie cookies immediately or store them in an airtight container.
