Danube Waves Cake (German Schneewittchentorte)

Donauwelle is a German and Austrian sheet cake. It's also known as Schneewittchenkuchen and Schneewittchentorte, which means Snow White cake, referring to the colours - black, white, and red. It consists of two pound cakes, one on the bottom, flavoured with vanilla, and one on top of it, flavoured with chocolate. Cherries, buttercream, and chocolate ganache are then added to the top, and as the cake bakes, the tart cherries will fall down through the batter, making a wavy pattern (maybe reminiscent of the Danube river?) in the cross-section of the cake.

Sour Cherries:
½ lb fresh sour cherries or from a jar, pitted (250 g)
1 Tbsp. Monk Fruit Sweetener with Erythritol (Granular)
1 Tbsp. Kirsch brandy
1 tsp. cornstarch

Cake:
1 cup + 1 Tbsp. all-purpose flour, unbleached (150 g)
1 tsp. baking powder
2/3 cup unsalted butter, softened at room temperature (150 g)
½ cup Monk Fruit Sweetener with Erythritol (Granular) (100 g)
1 tsp. vanilla paste
3 eggs
1½ Tbsp. unsweetened dutch process cocoa powder (10 g)

Vanilla Buttercream:
14 oz Pastry Cream (Crème Pâtissière) (400 g)
¾ cup unsalted butter, softened at room temperature (170 g)

Chocolate Glaze:
7/8 cup semi-sweet chocolate chips (150 g)
2 Tbsp. Canola oil

For the Sour Cherries:
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.

Feel free to use frozen pitted sour cherries too. Defrost and drain well before using it.

Make the Cake:
Preheat oven to at 350°F (175°C/155°C fan, Gas Mark 3). Prepare a 9x13x2 baking pan (25x30x5 cm). 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.

Pour half of the 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 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 Pastry Cream (Crème Pâtissière):
While the cake is baking, make the pastry cream. Or, 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.

Chocolate Glaze:
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.

To Serve:
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.