In a medium saucepan, whisk together the cornstarch and sugar until well combined and free of lumps. This prevents the cornstarch from clumping when you add the liquid and ensures even thickening throughout the filling. I like to press the mixture against the side of the pan with the back of a spoon to break up any stubborn clumps before proceeding.

Gradually pour in the water while stirring constantly to create a smooth slurry with the dry mixture—this prevents lumping. Add the blueberries and lemon juice, stirring gently to distribute the berries evenly throughout the mixture. The lemon juice will brighten the blueberry flavor and help balance the sweetness.

Place the saucepan over medium heat and stir frequently as the mixture heats. Cook for 8-12 minutes, until the filling comes to a gentle boil and thickens noticeably—you'll see it transition from a loose slurry to a glossy, coat-the-spoon consistency. Once thickened, remove from heat and stir in the salt and vanilla extract. I find that adding vanilla at the end preserves its delicate flavor rather than cooking it off.

Let the filling cool in the pan to room temperature, stirring occasionally to release steam and prevent a skin from forming on top. The filling will continue to thicken as it cools. Once cooled, it's ready to use in your pie or can be refrigerated until you're ready to assemble.