
Combine 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). Feel free to use your spoon or spatula to squish the berries against the side of the saucepan to encourage them to break down further.
Separately, in a small dish, whisk together cornstarch and 2 Tablespoons cold water until completely combined.
Add the cornstarch mixture into the blueberry mixture and stir well.
Stirring constantly, bring mixture to a rolling boil and then reduce heat and simmer, stirring frequently, until the sauce coats the bottom of a metal spoon.
Remove from heat, add butter and vanilla extract and stir until butter is melted.
Serve and enjoy. If your 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.