Preheat the oven to 325F.
Line a baking pan with parchment paper and spray with nonstick vegetable spray.
In a dry bowl, melt chocolate over barely simmering water. Once melted, let the chocolate cool a bit. Beat the butter and 1/2 cup of sugar until light and fluffy. Fold in the melted chocolate followed by the egg yolks, one at a time.
Whip the egg whites and a pinch of salt until bubbly, then slowly add 1/4 cup of sugar until the whites form stiff peaks.
Lighten your chocolate mixture by stirring in egg whites, a dollop at a time. Gently fold in the rest of the egg whites.
Pour the batter into a baking pan and bake for 10-15 minutes, depending on the size of the pan. The cake it ready when the sides pull away from the edges of the pan and the top of the cake is try to the touch. Once finished, remove from the oven and let cool for 10 minutes before inverting onto a wire rack.
Meanwhile, make the filling. Heat up heavy cream until it comes to a boil. Then pour over chopped chocolate. Let the mixture sit for a couple of minutes, then whisk until smooth. Let the mixture cool completely in the fridge, then take it out and whip until the filling doubles in volume.
Cut the sheet cake in half. Lightly spread one layer with the ganache filling and place the second half of the cake on top. Press down slightly and refrigerate.
Make the glaze: Place the chocolate and butter in a small bowl over simmering water. When the chocolate and butter are melted, remove from the heat, add water and corn syrup and whisk until smooth.
Set the rack holding the cake over a pan to catch the chocolate drippings. Pour glaze evenly over the cake. Allow cake to chill in the refrigerator for 30 minutes, until chocolate is set.