Melt the dark chocolate in short 30 second bursts in the microwave, stirring each time. Set aside to cool.
Cream the butter and sugar until lighter in colour and smooth.
Beat the egg yolks until they've doubled in volume.
Gradually add the egg yolks to the creamed butter and sugar, beating well.
Add the cooled chocolate and beat well to combine.
In a separate bowl, combine the flour, baking powder, bicarbonate of soda and salt.
In a jug, combine the milk and vanilla extract.
Add half the flour to the creamed mixture and beat well, then add half the milk and beat. Repeat with the remaining flour and milk.
In a separate bowl, beat the egg whites to soft peaks then fold the egg whites into the cake batter. Fold very gently so you don't knock the air out of the mixture!
Divide the mixture evenly between your tins. Please note: I don't grease and line my cake tins using the traditional method, I spray them with Wilton's Cake Easy and have never had a problem with cakes sticking.
If using shallow layer tins, your cakes will probably be baked after 20 minutes. If using 2 tins then they'll take around 30 minutes. The cakes are ready when a skewer inserted into the middle comes out clean.
Leave the cakes to completely cool while you make the buttercream.
To make the Nutella Buttercream
For the buttercream add the butter (make sure it's room temperature) to a stand mixer and beat it until it's creamy using the paddle attachment. This will take 3-5 minutes or longer if you're using a handheld electric whisk.
Add the icing sugar a tablespoon at a time (so that you don't end up enveloped in a cloud of icing sugar!) Beat for a few minutes until the icing sugar is completely incorporated.
Add a teaspoon of vanilla extract and 200g of Nutella to the buttercream and beat until fully combined.
If your buttercream is too thick add a tablespoon of milk or double cream if it's too thin add icing sugar a tablespoon at a time until it reaches the desired consistency.
Once your cakes have cooled you can turn them out from the tins and you're ready to assemble the Kinder Cake.
Begin by adding the first layer of chocolate sponge to a turntable or cake board. Spread with Nutella buttercream and repeat until the cake is fully assembled.
Use a piping bag (without a nozzle) or a palette knife to spread Nutella buttercream around the outside edges and top of your cake. I use a large cake spreader to get a smooth finish.
To make the White Chocolate Gold Drip
Fit a piping bag with a nozzle (I use Wilton 1M) and fill it with remaining buttercream. Pipe swirls around the top of the cake.
Add Kinder Chocolate decorations around the bottom and on top of the cake and your masterpiece is complete!