01 - Add the diced strawberries, granulated sugar, and lemon juice to a medium saucepan. Stir gently to coat the fruit evenly.
02 - Set the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until strawberries have softened and released their juices.
03 - In a small mixing bowl, whisk together cornstarch and water until completely smooth.
04 - Pour the cornstarch slurry into the saucepan with the cooked strawberries. Stir constantly and cook for an additional 3 to 5 minutes until the filling thickens and appears glossy.
05 - Remove saucepan from the heat. Stir in pure vanilla extract, if using, blending it thoroughly with the mixture.
06 - Allow the filling to cool completely before using. It will continue to thicken as it cools. Transfer to an airtight container and refrigerate for up to five days.