Mix the cornstarch and warm water in a small bowl. Warm strawberries and sugar in a saucepan over medium heat and bring to a simmer. Stir in the cornstarch mixture and simmer for 5 minutes. Remove from heat, stir in lemon zest, and cool completely.

Preheat oven to 350°F (177°C) and line a muffin pan with cupcake liners.

In a bowl, whisk flour, baking powder, baking soda, and salt. In a separate bowl, whisk melted butter and sugar, then add egg whites, sour cream, milk, and vanilla.

Combine wet and dry ingredients, mixing until smooth. Fill liners ⅔ full.

Bake for 20-22 minutes, until toothpick comes out clean. Cool for 10 minutes in the pan, then move to a wire rack.

Cut a small circle into the center of each cooled cupcake to make a pocket. Fill each cupcake with the cooled strawberry filling.

Whip heavy cream, sugar, and vanilla on medium-high speed until medium peaks form. Fold in reserved strawberry filling.

Frost the cooled cupcakes with whipped cream and garnish with sliced strawberries and fresh mint.

Serve immediately or store in the refrigerator.