Donauwelle (Danube Wave Cake) / Schneewittchenkuchen (Snow White Cake)

To start the German buttercream: In a large saucepan or small pot beat the egg yolks together, then slowly add the milk beating until smooth. Add the sugar and cornstarch and whisk until smooth.

Heat the pan over medium heat, whisking constantly until the mixture is very thick and easily coats the back of a spoon and/or reaches a temperature of 180°F/80°C with an instant read thermometer, about 10 to 15 minutes total.

Remove from the heat, whisk in vanilla until smooth, then use a rubber spatula to scrape the pastry cream mixture into a bowl. It will cool faster if it’s spread out in a wider bowl or container. Press a piece of plastic wrap directly onto the surface of the custard so it doesn't form a skin. Cool it to room temperature. (You can do this step up to 2 days in advance and refrigerate it until needed. In that case, bring it back to room temperature before using.)

To make the cake: Preheat the oven to 350°F / 180°C with the rack in the lower third of the oven. Line a 9-by-13-inch (24-by-32-cm) rectangular cake pan with parchment paper with some overhang around the sides to help you lift the cake out of the pan after assembling. Lightly grease the parchment.

Combine the flour, baking powder, and salt in a mixing bowl and set aside.

Add the butter and sugar to the bowl of an electric mixer fitted with the paddle attachment and beat on medium speed until light and fluffy (about 2 to 3 minutes). Add the eggs, one at a time, beating between additions until smooth. Scrape down the sides and beat in the vanilla.

Add the flour mixture in 2 additions and beat just until combined.

Evenly spread about ⅔ of the cake batter into the prepared pan.

Sift the cocoa powder into the remaining batter and add the milk. Mix until just combined, then carefully spread it over the top of the light-colored batter. The best way to do this is to dollop spoonfuls of chocolate batter over the vanilla layer and then carefully spread the batter until the chocolate layer is even. Do not swirl them together!

Evenly distribute the drained sour cherries over the batter. Bake for 28 to 32 minutes or until a toothpick inserted into the center comes out clean. Leave the cake in the pan but cool it completely before decorating, about 1 to 1 ½ hours.

To finish the German buttercream: The pastry cream and butter should both be at room temperature before proceeding with this next step. Add the butter to the bowl of an electric mixer fitted with the whisk attachment and beat until very light and creamy (about 2 minutes).

With the mixer running on medium speed, add the pastry cream a little at a time until completely combined. Scrape down the sides and then continue beating for another minute to ensure it’s smooth and creamy.

Spread the buttercream on top of the cooled cake, and cover with plastic wrap directly in contact with the buttercream. Chill the frosted cake for at least 1 hour or overnight.

To make the chocolate ganache: Add the chopped chocolate to a medium mixing bowl. Then heat the cream in a small saucepan over medium-high heat until steaming but not quite boiling. Pour the cream over the chocolate and let it sit for 2 minutes, then gently stir together until smooth.

To assemble the Donauwelle: Remove the plastic wrap from the cake and pour the ganache over the top. Instead of spreading it with a spatula, carefully tilt the cake from side to side as needed to ensure even coverage. Allow it to cool for about 10 to 15 minutes at room temperature, and if the cake was already relatively cold it shouldn’t take long for the ganache to start to thicken and set. (To test how much it has set, poke the ganache in one of the corners with the tip of a fork or knife to see if it leaves a clear impression. If it's still too soft, your waves will look messy.)

Once it has set enough, if desired, use a fork or a pastry comb to lightly drag over the top and create a wavy pattern.

Cool the cake completely in the fridge for about 30 to 45 minutes to allow the ganache to fully set.

Use the parchment paper overhang to carefully lift the cake out of the pan and set it on a large cutting board. Peel down the 4 sides of parchment paper to expose the edges of the cake. Cut into 15 rectangular slices (into thirds lengthwise and fifths crosswise). You may also cut the cake into smaller pieces if you prefer.

Refrigerate leftover cake slices for up to 3 days, but bring them back to room temperature before serving for the best texture.