Prepare the strawberry filling by mixing cornstarch and water until smooth.
In a saucepan, heat strawberries and sugar over medium heat, stirring and mashing slightly.
Bring to a simmer, then stir in the cornstarch mixture and cook for 5 minutes until thickened.
Remove from heat, add lemon zest, and let cool completely.
Preheat oven to 350°F (177°C) and line muffin tins.
Whisk flour, baking powder, baking soda, and salt in a bowl.
In another bowl, mix melted butter and sugar, then add egg whites, sour cream, milk, and vanilla.
Combine wet and dry ingredients until smooth.
Fill liners 2/3 full and bake for 20–22 minutes.
Cool completely before filling.
Cut a small center hole in each cupcake and fill with strawberry mixture.
Replace the top portion gently.
Whip cream, sugar, and vanilla until medium peaks form.
Fold in reserved strawberry filling.
Frost cupcakes generously with whipped cream.
Garnish and serve.
These Strawberry Shortcake Cupcakes are best enjoyed fresh for maximum flavor.