01 - In a medium saucepan, add diced strawberries, granulated sugar, and lemon juice. Stir to coat the strawberries uniformly.
02 - Place the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until strawberries release their juices and soften.
03 - In a small mixing bowl, whisk together cornstarch and water until a smooth slurry forms.
04 - Pour the cornstarch slurry into the strawberry mixture while stirring constantly. Continue cooking for 3 to 5 minutes, or until the filling thickens and develops a glossy finish.
05 - Remove saucepan from heat. Stir in vanilla extract, if desired.
06 - Allow the filling to cool entirely before using. The mixture will thicken further as it cools.
07 - Transfer cooled filling to an airtight container and refrigerate for up to 5 days.