This is a big cake with big flavor. It's ideal for feeding a crowd, particularly a crowd of chocolate-lovers. The soft, moist cake is nicely accented by the dense, fudgy icing, which makes it deserving of a tall glass of your coldest milk. We thank Food52 and Gibson2011 for the idea for this recipe.
Preheat the oven to 350°F. Grease a half-sheet pan (13" x 18"), line it with parchment, and grease the parchment.
To make the cake: Combine the chocolate, butter, oil, water, and cocoa in a medium saucepan or microwave-safe bowl.
Heat, stirring occasionally, until the mixture is smooth. Transfer it to a mixing bowl.
In a separate bowl, mix together the flour, sugar, baking soda, and salt until evenly combined.
Add the dry ingredients to the chocolate mixture, stirring until just combined.
Mix together the buttermilk, eggs, yolks, and vanilla, then add to the other ingredients, again mixing until just combined.
Pour the batter into the prepared pan, spreading it into an even layer.
Bake the cake for 18 to 20 minutes, until a cake tester inserted into the center comes out clean. Remove it from the oven, and allow it to cool completely in the pan.
To make the icing: Combine the chocolate, corn syrup, vanilla, and heavy cream in a microwave-safe bowl.
Microwave until the cream is very hot, and the chips have softened. Stir until the chips have melted and the mixture is smooth, reheating briefly if necessary.
Add the confectioners' sugar and cocoa, and beat until smooth.
Spread the icing evenly onto the cooled cake, and allow it to set for 30 to 60 minutes before serving.
