Crush oreo cookies in a blender or food processor. You'll need 1 cup for the cookie dough. Save the rest of the crushed cookies for rolling the cookie dough in and sprinkling over the frosting.
Whisk brownie mix and 1 cup of cookie crumbs together in a mixing bowl. Add melted butter, eggs and milk. Mix well.
Chill dough for 30 minutes in the fridge.
Scoop cookie dough using a medium cookie scoop. Roll cookie dough balls in crushed oreo cookies.
Place cookies on parchment paper lined baking sheets, 6-8 cookies per sheet.
Bake at 375 degrees for 10 minutes. Allow cookies to cool on baking sheets 7-10 minutes before transferring to wire rack.
While cookies cook, beat frosting ingredients together using an electric mixer.
Spoon frosting over cookies then sprinkle cookies with remaining cookie crumbs.