Scoop 2 1/2 cups each vanilla ice cream and coffee ice cream equally into 4 glasses (each 16 oz.).
Add 2 tablespoons hazelnut liqueur (1/2 cup total) or 1 tablespoon hazelnut syrup (1/4 cup total) to each glass, then slowly fill with cold coffee soda, about 2 bottles (12 oz. each) total.