In a heat-proof bowl, whisk the egg yolks and sugar until smooth and a bit lighter in color. Sift in the cornstarch and whisk vigorously until no lumps remain.

While whisking, slowly pour in ½ cup of the hot milk mixture into the egg yolks. Then, slowly whisk in the remaining hot milk mixture. Once combined, pour the mixture through a fine mesh strainer back into the saucepan.

Cook over medium heat, whisking continuously, until thickened and beginning to bubble (5 to 8 minutes). Remove from the heat and whisk in sea salt and the butter until melted.

Transfer the pastry cream to a bowl and press plastic wrap directly onto the surface to prevent skin from forming.

Chill for at least 2 hours or until ready to serve. The custard can be made up to 24 hours in advance. Whisk the pastry cream again until smooth before using, as it will have set into a firm custard.

While cream chills, Preheat oven to º400 and line 2 sheet pans with parchment paper. Roll out each puff pastry on a lightly floured surface, to approximately 11×16 inches. Transfer to lined sheet pan, top with another parchment paper, then an additional sheet pan, weighted with pie weights or dried beans. Bake for 20 minutes with pan and weights on top. Carefully remove top weighted sheet pan and top parchment and bake pastry, uncovered for another 7-9 minutes until golden brown. Remove from oven, cool 20 minutes.

While pastry bakes, slice 1 cup of the strawberries and puree, using an immersion blender or stand blender. Set aside. Chop remaining ½ cup of strawberries and set aside for topping.

Use a serrated knife to cut each pastry sheet into 15 roughly uniform squares. Stir 3 tablespoons strawberry puree into chilled pastry cream. Fill a plastic or piping bag with pastry cream and snip one corner of the bag.

Build mille-feuille: layer one pastry square with 2 rows of piped pastry cream, top with another pastry square, pipe 3 more rows of cream, top with final pastry square, dust with powdered sugar and top with chopped