01 - In a medium saucepan, mix diced strawberries, granulated sugar, and lemon juice until strawberries are evenly coated.
02 - Set saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until strawberries soften and release juice.
03 - In a small mixing bowl, whisk cornstarch with water until the mixture is completely smooth.
04 - Pour cornstarch slurry into the saucepan and stir constantly. Continue to cook for 3 to 5 minutes, until filling thickens and develops a glossy sheen.
05 - Remove from heat. Stir in vanilla extract if desired. Allow mixture to cool completely before using as a filling.
06 - Transfer cooled filling to an airtight container and store in the refrigerator for up to 5 days.