01 - In a medium saucepan, add diced strawberries, granulated sugar, and lemon juice. Stir thoroughly to evenly coat the fruit.
02 - Set the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until strawberries soften and begin releasing their juices.
03 - In a separate small bowl, whisk together cornstarch and water until fully blended and smooth.
04 - Slowly pour the cornstarch slurry into the saucepan, stirring constantly. Continue cooking while stirring for 3 to 5 minutes until the mixture thickens and develops a glossy appearance.
05 - Remove the saucepan from heat. Stir in vanilla extract if desired. Allow the filling to cool completely before application; it will continue to thicken as it cools.
06 - Transfer any leftover filling to an airtight container and refrigerate for up to 5 days.