Preheat your waffle iron according to the manufacturer's directions and set out small plates with sprinkles, mini chocolate chips, and chopped nuts for topping.
Prepare the brownie batter following the box instructions, but use a total of 3 eggs (one extra egg than the box calls for) and add the listed oil and water amounts from the ingredient list.
Lightly brush or spray the waffle iron with vegetable oil to prevent sticking.
Pour batter into the hot waffle maker and cook each waffle for about 3–5 minutes, or until set; repeat until all batter is used, brushing the iron as needed between batches.
Allow waffles to cool completely, then break each waffle into 4 pieces.
Place one piece of a brownie waffle on a surface, top with one scoop of ice cream, and use a spoon to spread the ice cream into an even layer.
Top with a second brownie waffle piece to make a sandwich and, if desired, press the sides of the ice cream into the prepared toppings on the plates.
Place assembled sandwiches on a tray and freeze for at least 2 hours, or until firm.
For longer storage, wrap sandwiches in plastic wrap and place in a freezer bag.