For the cake

1. Preheat the oven to 180 °C and grease the 2 baking tins. Cut out baking paper to the size of the bottom of the tin and stick it to the bottom of the cake pan. In the same way, cut some strips that you will stick around the inside edges. This should 100% ensure that the cakes come out of your tins in one piece!
2. Beat the margarine together with the eggs and mix further.
3. Add in the buttermilk and vanilla extract and continue mixing.
4. In a separate mixing bowl, add the dry ingredients: the self-raising flour, the Zùsto, the almond flour and the pinch of salt. Next, add this scoop by scoop to the rest of the mixture while continuing to stir.
5. Take a bowl and add the baking powder and apple cider vinegar to it. Mix them together until they begin to bubble and then add to the batter. Mix again briefly until it is mixed into the batter.
6. Divide the batter between the 2 baking pans. Tip: use bake-even strips if necessary. Place in the preheated oven for 30-40 mins (prick the cake once with a cake stick or skewer to check if the cakes are ready). Allow to cool in the tin for 15 mins before removing it from the tin (and continuing to allow it to cool on a wire rack).

For the Swiss buttercream

1. Chop the chocolate into equal pieces and place in a microwave-safe bowl. Melt the chocolate in the microwave on a low wattage (600 W) and melt for bursts of 30 seconds, stirring the chocolate between each burst. Continue until almost all the chocolate is melted. Once almost all of the chocolate has melted, keep stirring until even the final bits melt away. Set to one side. 
2. Put the egg whites together with the Zùsto in a bowl and heat in the bain-marie. Meanwhile, beat the egg mixture with a whisk. It might feel a little lumpy at first, but it will be OK! Once the Zùsto is completely melted, remove the bowl from the heat. This takes about 5 to 10 mins and you can check it by rubbing some egg white between your index finger and thumb. What you want, is for it to no longer feel ‘grainy’. If it does still grainy, continue to warm it.
3. Using a stand mixer or a hand mixer, beat the egg white-Zùsto mixture until stiff white peaks form (in other words, until you have meringue). While you are mixing it, it will gradually cool down. When the egg white is stiff, add in the butter one block at a time, making sure that you continue to mix it well between each addition (make sure your butter is at room temperature!). It’s very important that you do this slowly and don’t just suddenly dump all the butter in! At some point, the buttercream will appear to start to separating – keep calm and keep mixing! It will be fine!
4. Once the buttercream has a smooth texture, add in the melted white chocolate. Mix briefly again until the chocolate is fully mixed into the buttercream.

Putting together the cake

Would you like a rainbow on the cake as a cake topper? Then you will have to make sure you do this a day in advance.

Once the cakes have completely cooled, cut them in half (if the top has risen into a mound, cut it off so that you have ‘flat’ surfaces).

Make sure that you start stacking the cakes on a surface that you want to use for serving the cake (I use a cardboard cake board for this). This is important, because it will be difficult to move the cake around afterwards. First of all, spread a dollop of buttercream on the cake board and then place one of the cakes on top. Now spread a layer of buttercream between the cakes. Tip: use a cake bottom at the end (that way the top of your cake is flat). ‘Plaster’ the edges of the cake using a palette knife until they are smooth and you have a ‘naked’ cake. It’s called a crumb-free surface. Now place the cake in the refrigerator for 30 mins. You can also put the buttercream in the fridge along with the cake for 30 minutes, but make sure you don’t leave it in there for too long. If you do, it’s likely to become too hard and you will have to wait again (a long time!) before it softens enough to use for decorating. 

Next, you’re going to frost the cake. 

Then finish the cake with a cake topper, if you wish. Place in the refrigerator for at least 4 more hours before serving.