In a mixing bowl, combine all-purpose flour, brown sugar, and a pinch of salt. Gradually add softened unsalted butter, using your fingers or a pastry cutter to blend until the mixture forms a smooth dough. Roll the dough between two sheets of parchment paper to about ⅙ inch thick. Freeze the rolled dough for about 15 minutes, then cut out rounds using a small cookie cutter.

Preheat your oven to 355°F (180°C). In a saucepan, bring water, salt, and butter to a boil. Once boiling, quickly stir in the sifted all-purpose flour, mixing vigorously until the dough pulls away from the sides and forms a ball, about 2-3 minutes. Remove from heat and let it cool for about 5 minutes before adding room temperature eggs one at a time, mixing until smooth.

Line a baking tray with parchment paper and transfer the choux pastry into a piping bag fitted with a large round tip. Pipe small mounds about 2 inches apart, keeping them uniform. Take the frozen craquelin discs and place one atop each piped mound, pressing down gently to adhere.

Place the tray in the preheated oven and bake for 35-40 minutes, or until the choux are puffed and golden brown. Avoid opening the oven door during baking, as this can cause collapse. The pastries should sound hollow when tapped on the bottom when done. Remove from the oven and allow them to cool on a wire rack.

While the choux are cooling, prepare the vanilla bean pastry cream. In a saucepan, combine whole milk and vanilla bean, bringing it gently to a simmer. In another bowl, whisk together egg yolks, cornstarch, and sugar until well combined. Gradually pour the hot milk mixture into the egg mixture, whisking continuously, then return to the saucepan and cook until thickened, about 5 minutes.

Once the choux have completely cooled, fill a piping bag with the prepared pastry cream. You can poke a small hole in the side for piping or cut the choux in half like a sandwich. Fill each choux generously with pastry cream and place the tops back on. Serve immediately or refrigerate for up to 24 hours for the best flavor in your Choux au Craquelin.