Wash, hull, and dice the strawberries, then add them to a medium saucepan with lemon juice, cornstarch, and granulated sugar.
Cook on the stove over medium heat, stirring continuously, for about 10 minutes until the mixture thickens. Remove from heat and let cool while you prepare the cupcakes. Preheat oven to 350°F.
Line a muffin tin with cupcake liners (this recipe makes about 15 cupcakes, so use two tins or bake in batches).
In a medium bowl, sift together the flour, baking soda, and salt. Set aside.
In a large mixing bowl, whisk together the melted butter and granulated sugar. Whisk in the egg whites until well combined. Fold in the sour cream.
Alternate adding the flour mixture and milk to the wet ingredients: first half the flour, then half the milk, then the remaining flour and milk. Finally, fold in ½ cup of the strawberry filling.
Fill cupcake liners ¾ full and bake for about 20 minutes. Check for doneness with a toothpick—if it comes out clean or with moist crumbs, they’re ready. Let cupcakes cool completely.
Once cooled, use a piping tip to remove the centers, leaving about ½ inch at the bottom to hold the filling. Fill each cupcake with about a teaspoon of strawberry filling using a piping bag or spoon.
For the frosting, beat the butter and cream cheese with an electric mixer for 2 minutes. Add vanilla, a pinch of salt, and powdered sugar one cup at a time, scraping the bowl as needed. Mix in the remaining ¼ cup of strawberry filling and crushed freeze-dried strawberries (optional, for color).
Transfer frosting to a piping bag and frost the cupcakes. Garnish with halved strawberries if desired. Enjoy!