Whisk the cornstarch, water, and lemon juice together until all the cornstarch has dissolved. (I just use a fork to mix—very easy.) Set aside.

Warm the blueberries and sugar together in a small saucepan over medium heat. With a wooden spoon or sturdy silicone spatula, stir continuously for 3 minutes, gently smashing the berries against the sides of the pan to help them release some juices. (If you used frozen blueberries, you won’t need to smash them as much because they will naturally break down as they thaw over heat.) After 3 minutes, add the cornstarch mixture and continue to stir for another 2–3 minutes, smashing more berries if needed. The mixture will start to thicken.

Remove from heat and stir in the vanilla. Allow to cool. The mixture is liquid and drippy, but thickens considerably as it cools.

You can serve the sauce warm before it cools completely if desired, or store in the refrigerator for up to 1 week. Blueberry topping will be thick after refrigeration, so microwave for 15 seconds or warm on the stove to thin out, if desired.