01 - In a medium saucepan, combine diced strawberries, granulated sugar, and lemon juice. Mix thoroughly until the strawberries are well coated.
02 - Place the saucepan over medium heat and cook for 5 to 7 minutes, stirring occasionally, until the strawberries release their juices and begin to soften.
03 - In a small bowl, whisk cornstarch and water together until the mixture is completely smooth.
04 - Pour the cornstarch slurry into the saucepan. Continue stirring constantly and cook for 3 to 5 minutes until the filling thickens and develops a glossy finish.
05 - Remove the saucepan from the heat. Stir in vanilla extract, if using, and mix until fully incorporated.
06 - Allow the filling to cool completely before using it as a cake filling. For best consistency, the filling should thicken further as it cools. Transfer any leftovers to an airtight container and refrigerate for up to 5 days.