01 - In a medium saucepan, add diced strawberries, granulated sugar, and lemon juice. Stir until strawberries are evenly coated.
02 - Place saucepan over medium heat and cook for 5 to 7 minutes, stirring occasionally, until strawberries begin to release their juices and soften.
03 - In a small bowl, whisk cornstarch and water together until the mixture is smooth.
04 - Pour cornstarch mixture into saucepan. Stir continuously and cook for an additional 3 to 5 minutes until the filling thickens and appears glossy.
05 - Remove saucepan from heat. Stir in vanilla extract if desired.
06 - Allow filling to cool completely before using. It will continue to thicken as it cools.
07 - Transfer any remaining filling to an airtight container and refrigerate for up to 5 days.