Line a 9-inch square baking pan with parchment, leaving an overhang. Line the pan with 16 saltine crackers. Prepare an 18-ounce box dark chocolate brownie mix as the label directs; spread the batter over the crackers. Bake as directed; let cool. Top with 1/2 gallon softened ice cream and 16 more saltines. Freeze at least 4 hours or overnight. Remove from the pan using the parchment overhang and slice between the saltines.