Prepare the filling: In a medium saucepan, combine the sugar, cornstarch, and water. Stir until the cornstarch is dissolved. Cook over medium heat, stirring constantly, until the mixture thickens and comes to a boil (about 3-5 minutes).

Add flavoring: Remove from heat and stir in the lemon juice, vanilla extract, and butter (if using). Let the mixture cool for about 10-15 minutes.

Assemble the pie: Arrange the halved strawberries in the baked pie crust, placing them in a single layer for an even look. Pour the cooled syrup mixture over the strawberries, gently coating them.

Chill: Refrigerate the pie for at least 3-4 hours to set the filling.

Serve: Top with whipped cream just before serving, if desired.