01 - In a medium saucepan, add diced strawberries, granulated sugar, and lemon juice. Stir thoroughly to coat the fruit evenly.
02 - Set the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until the strawberries release their juices and begin to soften.
03 - In a small mixing bowl, whisk together cornstarch and water until the mixture is smooth and free of lumps.
04 - Pour the cornstarch slurry into the saucepan. Stir continuously and cook for an additional 3 to 5 minutes, until the mixture thickens and develops a glossy appearance.
05 - Remove the saucepan from heat. Stir in the vanilla extract, if using, until fully incorporated.
06 - Allow the filling to cool completely before using as a cake or pastry layer. The texture will become thicker as it cools. Store any remaining filling in an airtight container in the refrigerator for up to 5 days.