Whisk the granulated sugar, cocoa powder, and salt in a saucepan until combined.

Add the milk, butter, and corn syrup. Bring to a boil over medium heat stirring often.

Turn to low and boil for 1-2 minutes until the sugar is dissolved, stirring constantly.

Remove from heat and stir in the vanilla.

Cool slightly and transfer to a mason jar. Cool to room temperature with the lid off. Then, cover and store in the refrigerator for up to a week or in the freezer for up to three months.

The chocolate sauce will thicken as it cools. Reheat in the microwave.