Prepare strawberry filling by mixing diced strawberries with 6 tablespoons sugar, cornstarch, and warm water. Set aside to macerate.

Preheat oven to 375°F (190°C).

In a mixing bowl, combine flour, baking powder, baking soda, and salt. Set aside.

In another bowl, cream together 1 cup granulated sugar and melted butter. Add egg whites, sour cream, milk, and vanilla, then mix until combined.

Gradually add dry ingredients to wet ingredients, mixing just until combined.

Fill muffin tins with batter halfway, then add a spoonful of strawberry filling, and top with more batter until three-quarters full.

Bake in preheated oven for 22 minutes or until a toothpick comes out clean.

Allow muffins to cool before making whipped cream filling.

Whip cold heavy cream with 3 tablespoons sugar and 1 teaspoon vanilla until soft peaks form.

Cut a slit in each muffin and pipe in whipped cream filling.

Garnish with strawberry slices and fresh mint if desired.