Combine cold brew concentrate and sweetened condensed in a large measuring cup. Stir until combined and taste. If you prefer sweeter coffee, add condensed milk one tablespoon at a time until you have reached desired level of sweetness. Pour the mixture into silicone ice cube trays and place in the freezer until frozen. Um... That's it. When you're ready to serve, place one or two ice cubes in a glass, pour milk, and walk away. Allow the ice to melt slowly. Drink.