Place the frozen strawberries in a pot with the water, 1/2 cup of sugar and 2 drops of vanilla extract. Bring to a high boil. Turn off heat and cover with a lid.
In the meantime, bring the milk to a simmer with 2 drops of vanilla extract. Place the egg yolks in a bowl, add 1/2 cup of granulated sugar and whisk well until it becomes pale in color, then add the sifted flour and cornstarch. Pour the hot milk over the egg mixture slowly to temper, then place everything back in the pot. Cook on medium heat while mixing until it thickens. Transfer to another bowl and cover with a plastic film, let cool.
When the strawberries are thawed and slightly melted, remove the lid from the pot, put the flame on high and cook off the water until it becomes a thick puree, reserve and let cool.
When they are both cooled, mix the pastry cream and the strawberry puree together.
Prepare the souffle mold by buttering the inside and then coating with sugar. Make sure not to touch the inside once it's coated with sugar or the souffle batter will stick. Set aside until needed.
Place the egg whites in a bowl, add a small pinch of salt and beat up until it gets fluffy and white, add half the sugar. Whisk until it becomes very stiff but not too stiff and grainy. Add the remaining sugar and mix well. Fold the egg whites in a white pastry cream mixture delicately until well incorporated.
Place half the souffle batter in the buttered/sugared oven-proof dish, then the cubed sponge cakes and finish with the remaining souffle batter on top. Place the whole dish in another oven-proof pan filled with hot water (this is called the bain-marie cooking method).
Place in a preheated 400 degree oven and cook until the souffle rises and its top becomes slightly brown (approximately 10 to 15 minutes. The inside should still be slightly undercooked.) Remove and serve immediately. Decorate with fresh strawberries and a mint leaf. It is a good idea to serve with a strawberry or a vanilla sauce.