Heat the oven to 350°F (175°C) and line baking sheets with parchment paper. Stir the brownie mix and flour together in a large bowl.

Add the eggs and oil to the brownie mixture, then stir until a thick, sticky dough forms. Fold in the semisweet chocolate chips until they are evenly distributed.

Scoop approximately 1 1/2 tablespoons of dough per cookie onto the prepared baking sheets. Leave about 2 inches between portions to allow for spreading.

Bake for 8 to 10 minutes, until the edges look set while the centers remain soft and slightly underdone. Cool the cookies on the baking sheet for about 5 minutes, then transfer them to a rack to cool completely.

Beat the softened butter until smooth, then add the cocoa powder, confectioners' sugar, milk, vanilla, and salt. Beat until creamy, adding a few drops of milk if necessary.

Spread or pipe the frosting onto the completely cooled cookies. Let the buttercream sit briefly before serving if you prefer a slightly firmer finish.