How to Make It
Make the choux Preheat the oven to 375°. Line a large baking sheet with parchment paper.
In a medium saucepan, combine 1/3 cup of water with the milk, butter, granulated sugar and kosher salt and bring to a boil over moderate heat. When the butter melts, add the flour all at once and beat with a wooden spoon until a tight dough pulls away from the side of the pan, about 2 minutes.
Scrape the dough into a medium bowl. Using a wooden spoon, beat in the eggs one at a time until smooth. The dough should be glossy and fall slowly from the spoon in thick ribbons. Scoop the dough into a pastry bag fitted with a 1/2-inch plain tip. Pipe 1 1/2-inch mounds onto the prepared baking sheet, spacing them 1 inch apart. Brush the mounds with the beaten egg. Bake the choux for about 30 minutes, until browned and puffed; rotate the sheet halfway through baking. Let cool completely.
Meanwhile, make the filling In a medium saucepan, combine the cream, corn syrup and brown sugar and bring just to a boil, stirring. Keep warm over very low heat.
In another medium saucepan, cook 1/4 cup of the granulated sugar over high heat, without stirring, until it starts to caramelize, about 3 minutes. Gradually whisk in the remaining 1/4 cup of sugar, letting it start to caramelize before adding more. Cook, swirling the pan occasionally, until a deep amber caramel forms, about 3 minutes. Remove from the heat and carefully whisk in the warm cream mixture.
Bring the caramel to a boil over high heat and cook, whisking occasionally, until it reaches 228° on a candy thermometer, about 12 minutes. Remove from the heat and stir in the fleur de sel. Let cool completely. Transfer to a pastry bag fitted with a 1/4-inch plain tip. Refrigerate until just chilled, about 45 minutes.
Insert the piping tip in the bottom of each choux and pipe in the caramel filling.
Make the coating In a medium bowl, whisk the panko with the cocoa powder. Brush each choux with some of the beaten egg, then dredge in the panko and return to the baking sheet. Freeze overnight.
In a large saucepan, heat 3 inches of oil to 350°. In batches, fry the choux over moderate heat, turning, until browned and crisp, about 4 minutes per batch. Using a slotted spoon, transfer the falafel to paper towels to drain. Let stand for 10 minutes, then dust with confectioners’ sugar and serve.