To create this incredible Japanese Strawberry Sponge Cake, follow these simple steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and line two 6-inch round cake pans with parchment paper.

1. Prepare Strawberry Mixture: In a bowl, combine sliced strawberries and granulated sugar. Set aside to macerate.

1. Whisk Eggs and Sugar: In a large mixing bowl, whisk the eggs and granulated sugar until the mixture becomes pale and fluffy.

1. Sift Dry Ingredients: In another bowl, sift together the flour, cornstarch, and baking powder.

1. Combine Mixtures: Gently fold the dry ingredients into the egg mixture, ensuring not to deflate the batter.

1. Add Milk and Butter: Mix in the milk, melted butter, and vanilla extract until just combined.

1. Bake the Cakes: Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

1. Cool Cakes: Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

1. Prepare Whipped Cream: In a chilled bowl, beat heavy cream, powdered sugar, and vanilla extract until soft peaks form.

1. Assemble the Cake: Level the cooled cakes with a knife. Place one layer on a serving plate. Spread a layer of whipped cream and top with sliced strawberries. Place the second layer on top and repeat the process.

1. Finish with Cream: Use the remaining whipped cream to frost the sides and top of the cake. Decorate with fresh strawberries and optional mint leaves.

1. Chill Before Serving: Refrigerate the assembled cake for at least 30 minutes before slicing for a cleaner cut.