01 - In a medium saucepan, place diced strawberries, granulated sugar, and lemon juice. Mix 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 juices and soften.
03 - In a small bowl, whisk together cornstarch and water until smooth and free of lumps.
04 - Pour the cornstarch slurry into the saucepan while stirring constantly. Continue cooking for 3 to 5 minutes, or until the filling thickens to a glossy consistency.
05 - Remove from heat and incorporate vanilla extract, if using. Allow the filling to cool completely before using for best texture.
06 - Transfer any unused filling to an airtight container and refrigerate for up to five days.