Whisk sugar, cornstarch, and salt in a small bowl. Place blueberries in a medium saucepan over medium heat, then sprinkle the sugar mixture over the top. Add lemon juice and vanilla. Stir gently for about 1 minute until the berries are coated.

Bring to a boil over medium-high heat, stirring frequently. Reduce heat to medium-low and simmer for 5 minutes, until the juices turn clear and the filling coats the back of a spoon.

Remove from heat and let cool in the pan for 10 minutes. Spoon into a jar or bowl to cool completely; the filling will thicken further as it sits.