Prepare a 20 x 30 cm square baking sheet with some grease or parchment paper and preheat the oven to 180°C/350°F.
For the batter stir the ghee to a smooth cream. Add slowly the sugar and salt. The eggs have to be added one by one and stir well into the cream. Add the baking powder into the flour and then the flour in 2 or 3 steps into the batter.
Two third of the batter you put on the baking sheet. Spread it evenly. In the rest of the batter add the cocoa and milk. Stir well and spread the chocolate batter on the other. Now top the chocolate layer with the cherries, lightly pressed in the batter. Bake the cake in the lower third of the oven for 40 minutes.
Meanwhile prepare the vanilla pudding with the pudding powder, milk and sugar and let it cool.
When the cake is ready take it out of the oven and let it cool out.
Prepare the buttercream. Whisk the butter into a cream, add the cold vanilla pudding spoon by spoon. It's important that both have room temperature in order to combine well.
Put the buttercream on the top of the cake and leave the cake for an hour or more in the fridge.
For the icing melt the chocolate with the oil in a bain-Marie. Spread evenly the chocolate on the top of the cake and let it harden just a little bit to make a wavy design on the top. Put the cake back in the fridge and serve cold by cutting square pieces.
You can make also muffins with the Donauwelle. In this case make only thin layers in order to get a good size.