How to Make It
In a medium bowl, whisk the egg with the egg yolks, cornstarch and 1/2 cup of the milk until smooth. In a small saucepan, heat the remaining 1 1/2 cups of milk over low heat; keep warm.
In a medium saucepan, combine the sugar with 1/4 cup of water. Cook over moderate heat, swirling the pan and brushing down the side with a wet pastry brush, until the sugar dissolves. Cook undisturbed until a deep-amber caramel forms, about 5 minutes. Remove the saucepan from the heat and whisk in the butter; be careful, it will boil vigorously. While whisking constantly, slowly drizzle in the warm milk until blended; the mixture may separate a little. Slowly whisk in the egg mixture and bring to a boil, then cook over moderate heat, stirring, until thickened, about 2 minutes. Stir in the vanilla and sea salt. Strain the caramel custard through a fine sieve into a heatproof bowl. Press a piece of plastic wrap directly onto the surface of the custard and refrigerate until completely chilled.
Preheat the oven to 400º. Line a baking sheet with parchment paper or a silicone mat.
In a medium saucepan, combine the water, butter, sugar and salt. Cook over moderate heat for 1 minute, until the butter dissolves. Add all of the flour at once and stir vigorously with a wooden spoon, then cook until the dough pulls away from the side of the pan, 1 to 2 minutes. Remove the saucepan from the heat and let stand for 2 minutes, stirring a few times to cool down the dough. Add the eggs one at a time, beating constantly and thoroughly with a wooden spoon between additions: the dough should be smooth and shiny.
Transfer the dough to a pastry bag fitted with a 1/2-inch plain tip. Pipe the dough into twenty-four 1 1/2-inch mounds on the prepared baking sheet, spacing them 1 inch apart. Bake for about 25 minutes, until the tops and bottoms are golden. Transfer the baking sheet to a rack. Using a skewer, poke a small hole in the side of each cream puff to allow steam to escape; let cool completely.
In a medium saucepan, combine the half-and-half, cocoa, corn syrup and sugar. Bring to a boil over moderate heat, stirring occasionally. Remove from the heat. Stir in the butter and chocolate until melted and the sauce is smooth.
Scrape the custard into a piping bag fitted with a 1/4-inch plain tip. Pipe the cream into the hole in the side of each cream puff until full. Drizzle with some of the chocolate sauce, sprinkle with garnishes and serve.
These cream puffs could also be filled with Boursin or salmon spread for a savory hors d'oeuvre.