Combine 2 cups blueberries, 1/4 cup sugar, 1 tablespoon lemon juice, and 1 tablespoon water in a saucepan
Cook over medium heat, stirring occasionally
Mash some of the berries with a spoon as they soften
Simmer for 5 to 10 minutes until thickened
Remove from heat
Stir in 1/2 teaspoon vanilla extract if desired
Let cool before serving
Store in an airtight container in the refrigerator