01 - In a medium saucepan, add diced strawberries, granulated sugar, and lemon juice. Stir thoroughly until strawberries are evenly coated.
02 - Place saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until strawberries begin to soften and release juices.
03 - In a separate small bowl, whisk cornstarch with water until smooth.
04 - Pour cornstarch slurry into saucepan. Stir constantly and cook for 3 to 5 minutes, or until filling thickens and appears glossy.
05 - Remove saucepan from heat. Stir in vanilla extract, if desired.
06 - Allow filling to cool completely before layering. For best results, store leftovers in an airtight container in the refrigerator for up to 5 days.