01 - In a medium saucepan, mix the diced strawberries, granulated sugar, and lemon juice. Stir evenly to coat the fruit.
02 - Place the saucepan over medium heat. Cook, stirring occasionally, for 5 to 7 minutes until strawberries release juices and soften.
03 - In a small bowl, whisk cornstarch and water together until fully dissolved and smooth.
04 - Add cornstarch slurry to the saucepan. Stir constantly and cook for another 3 to 5 minutes until the mixture thickens and develops a glossy texture.
05 - Remove the saucepan from heat. Stir in pure vanilla extract if using. Allow the filling to cool completely before use; it will continue to thicken as it cools.
06 - Transfer cooled filling to an airtight container and refrigerate for up to five days.