Preheat oven to 325°C / 160°C and line two 8 inch cake pans.(For more information on making genoise, check out my genoise cake recipe post)
Put the eggs and sugar into a large heatproof bowl and whisk together
Place bowl over a pot of simmering water to warm the mixture while you whisk
Heat until all the sugar is dissolved and the mixture feels slightly warm to the touch (just above body temperature). If using a thermometer, you want it to reach 120°F / 40°C
Move the mixture to a stand mixture an whip on the highest setting for 5 full minutes
After 5 minutes, turn the mixer down to a medium speed and whip for a further 10 minutes
Sift flour into the mixture gradually and fold it in very gently, it's crucial you don't knock too much air out of the batter
Put oil and vanilla in a small bowl, then add a dollop of the cake batter to the oil (about the size of an apple). Mix it together thoroughly to incorporate the oil into the batter, then pour back into the cake batter and very gently fold in
Pour batter into cake pans and bake for 25-30 minutes or until the sponge is springy and bounces back when you press it
Remove from the oven and flip the pans upside down onto a cooling rack or piece of parchment paper and allow to cool entirely in this position. While it's cooling you can make the Chocolate German Buttercream
Assemble the Prinzregententorte
Once the sponges are completely cooled, cut them into extremely thin layers, as thin as you can get them. I find it easiest to use a cake slicer, but you could use a serrated knife
Take your prepared chocolate german buttercream and a small amount of it aside - enough to pipe rosettes on top of the cake at the end
Place a layer of sponge in the bottom of a tall cake pan, put some of the chocolate german buttercream on top and smooth it out evenly. You want to use just enough buttercream that you can achieve a layer roughly as thick as the sponge
Repeat until the last layer which should just be plain sponge with no buttercream on top. Traditionally, Prinzregententorte should have eight layers of sponge (mine only had six as I ran out of room in my cake pan!)
Place cake in the fridge to cool for 30 minutes
After 30 minutes, remove the cake from the pan carefully
Smooth a small amount of chocolate german buttercream all over the cake to lock in the crumbs and create a smooth base
Place cake in the fridge to chill until ready to decorate
Make the Ganache
Chop up chocolate and place in a bowl
Heat cream in a saucepan until scalding (tiny bubbles form before it gets to boiling temp)
Pour the cream over the chocolate and stir together until all the chocolate has melted and it is smooth and glossy
Add the butter and stir until combined
Let the mixture cool for about 15 minutes so it can reach a good pouring consistency. It should be slightly thicker but still liquid enough to pour smoothly. The ideal temperature is 89°F / 32°C if you are using a thermometer
Decorate the Prinzregententorte
Remove cake from the fridge and prepare it for ganache pouring. You can either keep the cake on a cardboard cake circle or the base of a cake pan and put it on a glass, or you could put the cake on a wire rack, you just want to be able to let the ganache drip down the sides of the cake without pooling at the bottom. Also make sure you have a sheet pan or something underneath to collect the ganache that drips
Pour the ganache over the cake, ensuring it covers all sides
Let it set up for about 15 minutes. About halfway through, you can mark the top of the cake with where your slices will be
Put the reserved chocolate german buttercream in a piping bag with a star nozzle and pipe a rosette on each slice of the cake
To slice, run a sharp knife under hot water to heat the metal, then wipe off with paper towel and make a steady cut into the cake. Repeat with every single slice you make so that the slices are clean and the layers stay defined