Add raspberries, sugar, lemon juice, and salt to a medium saucepan.
Cook berry mixture over medium heat until it comes to a boil. In a separate small bowl or cup, prepare corn starch slurry (water + cornstarch). Whisk until there are lumps. Add to saucepan with berries.
Reduce to low heat and let mixture simmer until the berries pop and soften. About 5-7 minutes.
Remove berries from heat. Using an emersion blender, puree berries until smooth. (You can also puree berries in a standard blender.)
*If you would like a smooth final produce without seeds, strain at this step. See blog for details*
Your raspberry puree is ready for use! The puree will thicken as it cools. This can be served warm or cooled. If using as a cake or cupcake filling, be sure to let it cool completely before use. Store in refrigerator in an airtight container for up to 2 weeks.