Combine berries, sugar, 2 tablespoons water, and salt in a small saucepan.
Bring to a gentle simmer over medium heat. Stir and mash until berries burst, 5–7 minutes.
Stir in lemon juice and vanilla. Cook 1 minute to brighten the flavor.
Whisk cornstarch with 1 tablespoon cold water. Stir in and simmer 30–60 seconds if thicker sauce is desired.
Strain through a fine-mesh sieve into a bowl. Cool, then refrigerate until chilled.