To a pitcher, add the coffee, hot water and sugar and stir to dissolve the coffee and sugar. Stir in the cold water and hazelnut liqueur.
When ready to serve, fill six 8-ounce mason jars halfway with ice and pour in the iced coffee mixture, filling two-thirds of the way to the top. Add milk, if using. Place a lid on and shake until frothy. Serve.
Any remaining coffee mixture will keep in the refrigerator up to 1 week.