01 - In a medium saucepan, add the diced strawberries, granulated sugar, and lemon juice. Stir until strawberries are evenly coated.
02 - Place the saucepan over medium heat and cook for 5 to 7 minutes, stirring occasionally, until strawberries release their juices and begin to soften.
03 - In a separate bowl, whisk together the cornstarch and water until fully smooth.
04 - Pour the cornstarch slurry into the saucepan. Stir continuously and cook for an additional 3 to 5 minutes, or until thickened and glossy.
05 - Remove from heat and stir in the vanilla extract, if using.
06 - Allow the filling to cool completely before filling cakes or other baked goods. The mixture will thicken further as it cools.
07 - Transfer any leftovers to an airtight container and refrigerate for up to 5 days.