In a large bowl, combine sugar, cocoa, and salt, and whisk to combine well. Store the mixture in an airtight container.
For individual servings, pour 1 cup whole milk into a microwave-safe mug, and microwave on high just until hot. Add 2 tablespoons of cocoa mix, and stir to dissolve. For a larger batch of cocoa, warm the milk in a saucepan set over medium-low heat, taking care not to let the milk boil; as it warms, stir in 2 tablespoons of mix for each cup of milk.
This mix will keep all winter if stored in an airtight container.