Wash, hull, and quarter the fresh strawberries.

Place the strawberries into a large slow cooker basin. Add the sugar, lemon juice, and salt. Stir well to combine.

Cover and cook on LOW for 4 hours until the fruit is very soft and has released its juices.

Remove the lid and use an immersion blender to carefully purée the hot mixture until completely smooth.

Stir in the vanilla extract. Prop the slow cooker lid slightly open using a wooden spoon to allow steam to escape.

Continue to cook on LOW for another 4 to 6 hours, stirring occasionally, until the mixture has reduced, darkened, and thickened to a butter consistency.

Test consistency by placing a spoonful on a cold plate; if it does not run, it is ready. Turn off the slow cooker.

Ladle into clean glass jars, cool to room temperature, and store in the refrigerator.