Preheat the oven to 350 degrees Fahrenheit. Line a cupcake pan with 12 cupcake liners.
In a medium bowl whisk together flour, cornstarch, baking powder, and salt. Set aside.
In a large mixing bowl beat the butter and sugar together until light and fluffy, about 2 to 3 minutes.
Add the egg and mix well. Add the egg whites and mix again until fully combined.
Stir in the vanilla extract.
Add half of the dry ingredients to the butter mixture and mix gently.
Add the sour cream and milk and mix until smooth.
Add the remaining dry ingredients and mix until just combined. Do not over mix.
Fold the chopped strawberries into the batter gently.
Fill each cupcake liner about two thirds full.
Bake for 18 to 20 minutes or until a toothpick inserted in the center comes out clean.
Let the cupcakes cool in the pan for 5 minutes. Transfer them to a wire rack to cool completely.
Dust with powdered sugar or top with whipped cream and fresh strawberries before serving.