In a mixing bowl, combine the sifted all-purpose flour, light brown sugar, and sea salt. Mix until well integrated. Add the room temperature butter, and with your hands or a scraper, blend everything. Aim for a uniform dough that’s soft but pliable.

Wrap the dough in parchment paper, shaping it into a flat rectangle. Chill it in the freezer for about 30 minutes. Chilling ensures the butter firms up, which is essential for the crackly texture we're after.

After chilling, take it out of the freezer and let it soften for a couple of minutes. Roll the Craquelin dough onto a sheet of parchment paper until it’s about 1/8 inch thick. Cut out circles using a cookie cutter or the mouth of a glass. These will sit atop your choux pastries.

Whisk together the egg yolks and egg in a bowl. Add the cornstarch and mix until smooth. In a saucepan, combine the granulated sugar with milk and salt. Heat the mixture over medium heat until it begins to simmer. Gradually pour a small amount into the egg mixture to temper it, whisking continuously. Then return the mixture to the saucepan and cook, stirring, until thickened. Remove from heat and stir in the vanilla, heavy cream, and butter. Let it cool, then chill.

In a separate bowl, combine the gelatin with the water and let it bloom for five minutes. Whip the cold heavy cream until soft peaks form. Gradually add the powdered sugar and vanilla. Gently fold in the gelatin mixture until fully combined. For increased stability, consider adding mascarpone cheese.

Cook your choux pastry mixture according to your preferred recipe. Once baked and cooled, use a piping bag to fill them with the salted caramel diplomat cream.

Place the cut craquelin discs on top of each filled pastry. Bake again according to your choux pastry recipe's instructions until golden and crisp.