Melt butter in a medium sauce pan over medium heat.
Add berries, sweetener, egg yolks, and salt. Heat until all sweetener has dissolved and the berries pop.
Lower heat and cook on low until the mixture has thickened.
Stir in the lemon juice.
Press the mixture through a fine mesh strainer over a small bowl. Press out as much liquid as possible, leaving pulp and seeds in the strainer.
You can pour the mixture into a different bowl on covered container and allow to cool for 2-3 hours before serving. It will thicken further as it cools.