Combine 2 ½ cups (350g) blueberries, ¼ cup water, sugar, and lemon juice in a medium saucepan over medium-high heat.
Stir frequently until the blueberries release their juices (about 5-7 minutes).
In a small bowl, whisk together cornstarch and remaining 2 Tablespoons cold water until completely combined.
Pour into blueberry mixture and stir well.
Stirring constantly, bring mixture to a rolling boil and then reduce heat and simmer, stirring frequently, until blueberry sauce coats the bottom of a metal spoon.
Remove from heat, stir in butter and vanilla extract (if using), until butter is melted. Add remaining 1 ½ cups (210g) blueberries and stir well.
Serve and enjoy. If your blueberry sauce becomes too thick you can always add water while the mixture is still hot to make it thinner, but be sure to only add a teaspoon or two at a time until desired consistency is reached. Blueberry sauce is best served warm.
*A little vanilla extract goes a long way in this recipe, it enhances the sweetness of the blueberry sauce. For a more tart sauce with a stronger blueberry flavor, omit the vanilla extract.Store unused blueberry sauce in the refrigerator in an airtight container. Blueberry sauce will keep at least a week in the refrigerator (many sources say longer but I've never kept mine longer than a week).