Combine raspberries water, sugar, lemon juice, cornstarch, and salt in a medium-sized saucepan over medium-low heat.
Cook, stirring frequently, until berries release their juices and mixture becomes very liquidy.
Increase heat to medium and stir constantly, bring to a simmer.
Simmer, stirring constantly for about 1 minute, until berries are broken down, deeper in color, and the mixture is slightly thickened (it should coat the back of a spoon).
Remove from heat and immediately stir in butter.
If you want a seedless sauce, pour immediately through a fine-mesh strainer and discard seeds.
Allow to cool to room temperature, then pour into a container, cover, and refrigerate until ready to use.
