Set aside 1/3 of the Blueberries. Set aside 1/3(4 ounces/113 grams) of the fresh blueberries for later.
Combine. Add the remaining 2/3 blueberries, the sugar, water, lemon juice, cornstarch, and a pinch of salt to a medium saucepan.
Cook to Soften. Cook over medium heat, stirring with a spatula, until the blueberries soften and the sauce thickens, about 5-8 minutes. (The sauce will go from opaque to translucent and glossy.) It's ok to gently break up some of the berries with your spatula as they cook, if you like.
Stir in Remaining Blueberries. Add the reserved 1/3 of the blueberries, along with the vanilla extract and cook 2-3 minutes more.
Cool To Set. Let the blueberry sauce cool a bit, then transfer to an airtight container or jar and chill in the refrigerator. The blueberry sauce will thicken up even more in the refrigerator. The ideal texture for using on cake or cheesecake is scoop-able, but not runny. If you want to thin it out more (for ice cream, oatmeal, etc.), you can stir in 1 teaspoon of water at a time until you reach your desired consistency.
Serve & Store. Enjoy on cheesecake, pound cake, ice cream, oatmeal, yogurt, pancakes, waffles, and more! Store leftover blueberry topping in an airtight container in the fridge up to 4-5 days. (This blueberry sauce can be frozen, but the texture will soften more upon thawing.)