01 - In a medium saucepan, add diced strawberries, granulated sugar, and lemon juice. Stir to ensure strawberries are evenly coated.
02 - Place the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until strawberries release juices and begin to soften.
03 - In a small bowl, whisk together cornstarch and water until fully smooth.
04 - Add the cornstarch mixture to the saucepan. Stir constantly and cook for an additional 3 to 5 minutes, or until the filling thickens and becomes glossy.
05 - Remove saucepan from heat. Stir in vanilla extract, if using.
06 - Allow filling to cool thoroughly before using in cakes or pastries; it will continue to firm as it cools.
07 - Transfer cooled filling to an airtight container and refrigerate for up to five days.