01 - Place the diced strawberries, granulated sugar, and lemon juice in a medium saucepan. Stir to blend evenly.
02 - Set the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until the strawberries release juices and soften slightly.
03 - In a small mixing bowl, whisk together cornstarch and water until completely smooth.
04 - Pour the cornstarch slurry into the saucepan. Stir constantly and cook for 3 to 5 minutes, until the filling thickens and appears glossy.
05 - Remove the saucepan from heat. Stir in vanilla extract, if desired. Let the filling cool completely before use. It will continue to thicken as it cools.
06 - Transfer any leftover filling to an airtight container and refrigerate for up to 5 days.