In a small bowl, mix cornstarch and warm water until smooth.

Add strawberries and sugar to a saucepan over medium heat.

Cook while stirring and gently crushing some strawberries.

Bring to a simmer.

Stir in the cornstarch mixture.

Continue simmering for 5 minutes, stirring constantly.

Remove from heat and stir in lemon zest.

Transfer to a bowl and cool completely.

Preheat oven to 350°F (177°C).

Line a 12-cup muffin pan plus 3 additional liners.

In a large bowl, whisk flour, baking powder, baking soda, and salt.

In another bowl, whisk melted butter and sugar together.

Add egg whites, sour cream, milk, and vanilla.

Mix until fully combined.

Pour wet ingredients into dry ingredients.

Whisk until smooth and creamy.

Bake for 20–22 minutes or until a toothpick comes out clean.

Cool in the pan for 10 minutes.

Transfer to a wire rack and cool completely.

Fill the Cupcakes

Reserve ½ cup of strawberry filling for the whipped cream.

Cut a small circle in the center of each cooled cupcake.

Remove the center piece carefully.

Spoon 1–2 teaspoons of strawberry filling into each cupcake.

Trim the pointed end from the removed cupcake piece.

Place the top piece back over the filling.

Using a mixer, whip heavy cream, sugar, and vanilla until medium peaks form.

Gently fold in the reserved strawberry filling.

Use immediately or refrigerate for up to 24 hours.

Frost each cupcake generously with strawberry whipped cream.

Garnish with fresh strawberry slices and mint leaves.

Serve immediately or refrigerate until ready to serve.