Step 1Set a rack in the middle level of the oven and heat the oven to 350 degrees. Bring a small pan of water to a boil and turn off the heat. Combine the chocolate and one-fourth cup water in a heatproof bowl and place it over the pot of water. Stir until the chocolate is melted and smooth. Keep warm.
Step 2For the cake batter, put the 6 egg yolks in the bowl of an electric mixer and whisk in one-fourth cup of the sugar by hand. Place the bowl on the mixer fitted with the whisk attachment and whip the mixture on medium-high speed until the yolks and sugar are light and thickened, about 2 minutes. If you only have one mixing bowl and one whisk, scrape the yolk mixture into another bowl. Wash the whisk and bowl in soapy water, then rinse and dry them.
Step 3Put the 6 egg whites and salt in a clean, dry mixer bowl. Place on the mixer fitted with the whisk attachment, and whip the whites on medium speed until they are white, opaque and beginning to hold a very soft peak. Increase the speed to medium high and whip in the remaining one-fourth cup sugar in a stream. Continue whipping the whites until they hold a firm peak.
Step 4Stir the melted chocolate into the yolk mixture, immediately followed by one-fourth of the whipped egg whites. Sift and fold in the flour, and then fold in the remaining egg whites.
Step 5Scrape the batter into a 12-by-18-inch jellyroll pan, bottom and sides buttered and lined with buttered parchment or foil. Use a spatula to spread the batter evenly.
Step 6Bake the cake layer until it is risen (it will not rise very high) and firm when pressed with a fingertip, about 15 or 20 minutes. Use a spatula under the parchment to help lift the cake from the pan onto a wire rack to cool.
Step 1For the filling, bring the cream and corn syrup to a boil in a large saucepan, stirring occasionally to make sure that the corn syrup doesn't fall to the bottom and burn. Remove the pan from the heat and add the butter and chocolate. Gently shake the pan to make sure that the butter and chocolate are submerged, then wait 2 minutes for them to melt. Whisk the filling smooth and pour it into a bowl. Refrigerate the filling until it is about 80 degrees, or until it is of spreading consistency, about 45 minutes. Don't leave the filling in the refrigerator too long or it will become too hard to whip.
Step 2To assemble the cake, slide the paper with the cake still stuck to it onto a cutting board. Cut through both the paper and the cake with a sharp, serrated knife to make two 12-by-9-inch rectangles. Slide one off the cutting board.
Step 3Scrape the cooled filling into the bowl of an electric mixer and beat it with the paddle on medium speed until it is lightened to a milk chocolate color. Don't over-beat or the filling will separate.
Step 4Immediately use an offset spatula to spread the whipped filling evenly over the cake layer on the cutting board.
Step 5Invert the remaining cake layer, still stuck to the paper for easy handling, to a cookie sheet with no sides or to a stiff rectangular piece of cardboard. Gently slide the cake layer into place over the filling. Carefully peel off the paper.
Step 6Place a stiff cardboard or cookie sheet on the cake and gently press to make sure the top layer of cake adheres well to the filling. Refrigerate the cake while preparing the glaze.
Step 1For the glaze, bring the cream and corn syrup to a simmer in a medium saucepan, stirring occasionally. Remove the pan from the heat and add the chocolate. Gently shake the pan, to make sure that all the chocolate is submerged, then wait 2 minutes for it to melt. Whisk the glaze just until smooth. Avoid whisking too much or the glaze will be riddled with bubbles. Let the glaze cool until it is just slightly warm to the touch, about 100 to 105 degrees.
Step 2After the glaze has cooled, remove the cake from the refrigerator and pour the glaze over it. Quickly spread the glaze evenly with a medium offset spatula. Don't worry about any glaze that drips down the sides of the cake; it will be trimmed away later.
Step 3Refrigerate the cake again for at least 1 hour, or up to 24 hours, to set the glaze.
Step 4Rinse a long, sharp, serrated knife in hot water, then wipe it dry with a cloth. Use the knife to trim the sides of the cake evenly, rinsing and wiping after each cut. Cut the cake into 1 1/2 -inch squares. Keep the cake refrigerated, but bring to room temperature for an hour or so before serving. Cover leftovers with plastic wrap and refrigerate. Bring to room temperature again before serving.