Preheat oven to 350°F. Line 2 (8-inch) square baking pans with parchment paper, allowing 2 to 3 inches to extend over sides; spray parchment paper with cooking spray.
Prepare brownie batter according to the package directions. Divide the batter evenly among the two pans and spread with the back of a spoon or spatula into an even layer. If you are decorating or adding any other candies (i.e. M&M’s or sprinkles) to the brownies, sprinkle on one pan evenly. See notes for alternative methods for decorating!
Bake both pans in the oven on a middle rack and cook for about 18-22 minutes. Visually the edges will start to form a crust and the middle should look matte, no longer shiny. if you stick a tooth pick in the center, it should not look wet and underdone but some crumbs may stick, which means they are done baking. You don’t want to over cook them. Let cool in pan on a wire rack for 5 minutes.
Place brownies in pans, uncovered, into the freezer until completely cool, 30 minutes to 1 hour. If you make the brownies ahead of time (before you plan on assembling) you can let them cool at room temperature. When you do go to assemble, I still suggest freezing for 20-30 minutes to make it easier to lift out of the pans without them breaking.
When just about ready to assemble take the ice cream out of the freezer to soften slightly, 5-10 minutes.
Once ice cream is soft, spread in an even layer over the top of 1 cooled brownie pan. If you added candy to one brownie pan use that as the top layer. Remove brownie from the second pan and gently peel the parchment paper off the back. Place the second brownie on ice cream layer, top side up, lightly pressing down to seal. Freeze until firm, about 1-4 hours pending the kind of ice cream you purchased.
Once firm, run a knife along the edges to loosen the ice cream sandwiches. Using the parchment paper as handles, remove the sandwiches from the pan and place on a cutting board.
Trim away excess ice cream that may have oozed out. Cut brownies into rectangles or squares. They are very rich so small sizes are best. Serve immediately or place in an airtight container and place back in the freezer until ready to eat. You can also wrap each sandwich individually with plastic wrap to keep them more fresh.