Mix all the ingredients together until they are combined and there are no lumps.
Add the hot water and mix in until totally combined.
⅓ cup Hot Water
Divide the batter between the cake pans and bake for 30-40 minutes or until a skewer comes out clean.
Make the Chocolate Orange Filling
Zest the orange and place it into a saucepan along with the milk, vanilla and two tablespoons of the sugar. Save the leftover orange for use in the frosting later.
2 ½ cups Whole Milk, 1 Orange, 2 teaspoon Vanilla
Add the egg yolks to a large mixing bowl and whisk together with the remaining sugar, flour and cornflour. Set aside.
½ cup White Sugar, 5 Egg Yolks, 1 tablespoon All-purpose flour, 2 tablespoon Corn starch
Put the saucepan with the milk mixture on a low heat and let it slowly warm up
Once the milk mixture starts to feel warm, pour a little of it into the egg yolk mixture and whisk it in.
Add a little more milk to the egg yolk mixture and whisk it in completely. Continue adding in small increments until all the milk is incorporated into the egg yolks
Once all the milk is added, pour the entire mixture back into the saucepan and continue to heat, whisking constantly until the mixture thickens.
You'll know the custard is ready when it can coat the back of a spoon and you can run your finger down the spoon and leave a trail there. It's super important that you whisk the custard the whole time it is cooking and don't step away, as the thickening part happens in a matter of seconds and if you don't whisk you may end up with lumps.
Remove from the heat and pass the custard through a sieve into a medium bowl to remove the pieces of orange zest and any lumps.
Melt your dark chocolate in the microwave in 30-second increments, then pour the melted chocolate into the orange custard and stir until it is completely smooth and incorporated.
Cover the chocolate orange custard in plastic wrap, making sure it touches the surface of the custard so that a skin cannot form, and let it cool completely (about 2 hours).
Once the chocolate orange custard has completely cooled you can start making the final filling.
Add the room temperature butter and icing sugar to the bowl of a stand mixer and beat until pale and creamy (about 1-2 minutes).
3 sticks Butter, 1 ½ cups Icing Sugar
Add the cocoa powder and 2-3 tablespoons of juice from the orange you zested earlier and beat until everything is combined.
⅓ cup Cocoa Powder
Switch to a whisk attachment and on a medium speed, add the cooled chocolate orange custard one tablespoon at a time, letting it whip for a few seconds in between additions.
It should be beautiful and thick and silky, but if it doesn't look quite right, let it whip for another minute on high speed.
Make the Chocolate Ganache (optional)
Pour your chocolate pieces into a bowl and set aside.
1 cup Chopped Chocolate
Zest one orange and then peel the zested peel from the orange and set aside.
1 Orange
Add the whipping cream to a small saucepan along with the orange zest and the orange peel.
½ cup Whipping Cream
Heat over a low heat until the cream just begins to bubble slightly.
Pour the hot cream through a sieve and onto the chopped chocolate, this will remove the orange zest and peel.
Let the chocolate mixture sit for a minute or so, then stir it together until everything is glossy and free from lumps.
Allow to cool and thicken before using. See notes on decorating the cake anove for when you should use the ganache depending on how you want to decorate.
Notes
Make sure you heat the milk for the custard slowly to allow the orange time to infuse properly. Once the custard starts to heat, make sure you are whisking constantly to avoid getting any lumps.