01 - In a medium saucepan, add the diced strawberries, granulated sugar, and lemon juice. Stir thoroughly to coat the strawberries.
02 - Set the saucepan over medium heat and cook for 5 to 7 minutes, stirring occasionally, until the strawberries release their juices and soften.
03 - In a small mixing bowl, whisk the cornstarch and water until a smooth slurry forms.
04 - Pour the cornstarch slurry into the saucepan. Stir constantly and cook for 3 to 5 minutes, or until the mixture thickens and turns glossy.
05 - Remove from heat. Stir in the vanilla extract, if using.
06 - Allow the filling to cool completely before using. The mixture will thicken further as it cools.
07 - Transfer any leftover filling to an airtight container and refrigerate for up to 5 days.