Prepare the filling least six hours before baking the cake.

Put half the chopped chocolate into a mixing bowl. In a separate saucepan, bring half the heavy cream to a simmer. Pour the hot cream onto the chocolate, letting the chocolate melt. Whisk it until smooth and well incorporated.

Refrigerate until it cools and thickens to the consistency of a spreadable icing.

Preheat the oven to 350 degrees. Butter an 8-inch round cake pan and line it with buttered parchment paper. Don't butter the sides.

Set a metal mixing bowl over a saucepan half filled with simmering water. The water shouldn’t touch the bottom of the mixing bowl. Add the egg yolks, eggs, and sugar to the mixing bowl, and whisk together, allowing the heat from the simmering water to warm the mixture. Keep whisking the egg mixture until it reaches the ribbon stage, tripling in volume.

In a separate bowl, sift the flour and cocoa together. Gently sprinkle the flour and cocoa mixture onto the batter and fold the mixture in 3 additions.

Take a cup of the batter, mix it with the butter, and then gently fold in the butter mixture back into the main batter.

Spoon the cake batter into the prepared cake pan, and bake for 20 minutes, or until the cake springs back slightly when pressed. The cake should have risen to the top of the pan. Remove the cake from the oven onto a cooling rack. I like to invert to help the sponge expand.

Put the remaining half of the chopped chocolate into a mixing bowl. In a separate saucepan, bring the remaining heavy cream to a simmer. Pour the hot cream onto the chocolate, letting the chocolate melt. Whisk it until smooth and well incorporated, and then set it aside while you assemble the cake.

Slice the cooled sponge cake into three horizontal layers. Place the bottom layer on a cooling rack. which will allow the topping to flow below. Using the refrigerated icing, top the bottom discs with a thick layer of icing, then add the next layer and repeat the with the filling. Place the top of the cake on the second layer of icing. I like to use remaining frosting to fill in any gaps between layers.

Ladle the warm chocolate icing over the top of the sponge cake, allowing it to run down the sides.

If you wish to add further decorations, allow the cake to cool for two hours so the ganache has had a chance to set.