meanwhile, make the filling
In a large saucepan, combine 2 1/2 cups of the water with the sugar, corn syrup and cocoa powder and bring to a boil, whisking constantly. In a bowl, whisk the cornstarch with the remaining 1/2 cup of water until smooth; whisk into the cocoa mixture. Cook over moderately high heat, whisking constantly, until very thick, 3 minutes. Off the heat, whisk in the butter, vanilla and salt. Scrape the filling into a bowl and press a sheet of plastic wrap directly onto the surface of the filling (you should have about 5 cups). Let cool, then refrigerate until firm, 45 minutes.