Combine rasperries, sugar, lemon juice, and salt in a small saucepan set over low heat. Cook until berries release their juices and just start to break down, about 5 minutes. Using a rubber spatula, press berries through a fine-mesh sieve; discard solids. Return strained juice to saucepan, stir in cornstarch, and cook until mixture comes to a boil. Let cool. Store, refrigerated, in an airtight container for up to 3 days.