In a saucepan, whisk together the sugar, cocoa, and flour until all lumps are removed.
Whisk in the salt and milk, and butter, and bring to a boil over medium-medium high heat.
Reduce heat to low; stirring constantly. Simmer for about 30 seconds and then remove from heat. Stir in the vanilla then let the sauce cool. The sauce will be very thin while warm but will thicken quite a bit as it cools.
Pour into glass jars then store in the refrigerator for up to 2 weeks. Simply reheat in the microwave for a few seconds before serving.