01 - In a medium saucepan, toss together the diced strawberries, granulated sugar, and lemon juice until the strawberries are evenly coated.
02 - Set the saucepan over medium heat and cook for 5 to 7 minutes, occasionally stirring, until the strawberries release juices and begin to soften.
03 - In a small bowl, whisk together the cornstarch and water until smooth.
04 - Pour the cornstarch mixture into the saucepan, stirring constantly. Cook for an additional 3 to 5 minutes until the mixture thickens and acquires a glossy appearance.
05 - Remove from heat. Stir in the vanilla extract if desired.
06 - Allow the filling to cool completely before using. The consistency will continue to firm up as it cools.
07 - Store any unused filling in an airtight container in the refrigerator for up to 5 days.