1. For 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.
3. Scrape 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.
4. Immediately use an offset spatula to spread the whipped filling evenly over the cake layer on the cutting board.
5. Invert 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.
6. Place 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.
Glaze and assembly
1/2 cup heavy whipping cream
2 tablespoons light corn syrup
6 ounces semisweet chocolate, cut into 1/4 -inch pieces
1. For 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.
2. After 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.
3. Refrigerate the cake again for at least 1 hour, or up to 24 hours, to set the glaze.
4. Rinse 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.
Each serving: 247 calories; 4 grams protein; 21 grams carbohydrates; 1 gram fiber; 17 grams fat; 4 grams saturated fat; 66 mg. cholesterol; 27 mg. sodium.