Add pitted sour cherries to a bowl along with sugar, Kirsch (optional), and cornstarch. Stir to combine and set aside until needed. If you are using sour cherries from the jar, you can leave this step out, just make sure to drain the cherries well before using.TipFeel free to use frozen pitted sour cherries too. Defrost and drain well before using it.
make the cake
Prepare a baking pan approx (25 cm x 30 cm and 5 cm high OR 9-inch x 13-inch x 2-inch high). Line with parchment paper and grease on all sides. In a small bowl, stir to combine all-purpose flour, baking powder, and a pinch of salt. In a large bowl, cream the softened butter with the sugar, and vanilla paste until pale, fluffy, and creamy, for about 5 minutes on high speed using an electric mixer or stand mixer. Gradually add the eggs. Add the next egg once the previous one is completely incorporated. Add the flour mixture and mix it in quickly, using an electric mixer or stand mixer. Don't overbeat the mixture.
spread the cake over the baking pan
Pour half of the prepared cake batter into the prepared baking pan. Spread evenly using a spatula. Fold the unsweetened cacao powder into the rest of the cake batter to get an even brown cake batter. Pour the brown cake batter over the cake batter in the pan. Spread evenly using a spatula. Arrange the drained sour cherries on top. Place in the preheated oven on the middle rack. Bake for 25 - 30 minutes at 180 °C / 350 °F or until it is fluffy, yet firm when tested with your fingers. Remove the baked cake batter from the oven and set aside to cool.
make the buttercream
While the cake is baking, make the pastry cream (click for the recipe). Make half of the pastry cream. You can make this step one day ahead and keep it chilled (covered with cling film) in the fridge until needed, but at least for one hour. Ten minutes before using, bring to room temperature. Add softened butter to a bowl. Cream the butter using an electric mixer. Mix for about 5 - 8 minutes or until creamy, fluffy and smooth. Working in batches of three, incorporated the creamed butter into the pastry cream. When all of the butter is incorporated, continue to mix for 3 - 4 minutes or until the cream is smooth and creamy. Spread the buttercream evenly over the chilled cake using a spatula. Place in the fridge for at least one hour or overnight.TipFeel free to use a different pastry cream recipe. You will need 400g or 14 ounces of pastry cream.
Melt the chocolate chips and stir in the canola oil. While the chocolate is still slightly warm (but not hot), pour it over the chilled buttercream. Working fast, move the baking pan around so that the chocolate spreads evenly. Place in the fridge for one hour for the chocolate to set.
Cut the chilled Donauwelle into 15 slices and serve. Keep chilled in the fridge for up to 3 days. They are best served at room temperature.