Preheat oven per brownie mix directions and line two baking sheets with parchment.
Mix brownie batter according to package, fold in cocoa and chips if using.
Drop tablespoon-sized scoops 1.5 inches apart to make small cookie rounds.
Bake 8–10 minutes until edges set but centers still soft; cool 5 minutes on sheet.
Let cookies cool completely on a rack so they don't melt ice cream too fast.
Scoop a small scoop of ice cream onto a cookie, top with another, press gently, then roll edges in powdered sugar if desired.
Freeze assembled sandwiches on a tray for 30–60 minutes to firm before serving.