Danube Wave Sheet Cake (Donauwelle)

Ingredients

¾ cup plus 3 tbsp220 mloilNeutral-tasting
2 sticks225 gunsalted butterSoftened. German butter is unsalted and has about 82 percent fat. In the US use unsalted butter; European-style unsalted butter is closest. In the UK use unsalted butter.
2 cups420 ggranulated sugar
2 tspvanilla sugar1 packet = 2 tsp (8 g). Sugar flavored with vanillin (artificial vanilla), sold in small packets of about 8 g. In the US use 1 tsp vanilla extract plus 2 tsp sugar for each 8 g packet; in the UK, vanilla sugar sachets, or 1 tsp vanilla extract plus 2 tsp sugar per sachet.
1 pinchsalt
7large eggsGerman size M
3 ¼ cups400 gall-purpose flourIn German recipes plain flour means soft wheat flour Type 405 unless stated otherwise. In the US use all-purpose flour; in the UK, plain flour.
1 tspbaking powderGerman baking powder is sold in 15 g packets, each meant for about 500 g (4 cups) of flour. In the US and the UK use baking powder, 1 tbsp plus ¾ tsp per 15 g packet.
5 oz150 graspberriesFresh or frozen

2 cups500 mlmilk
⅔ cup140 ggranulated sugar
3egg yolks
5 tbsp40 gcornstarchOr custard pudding powder. Cornstarch is a fine starch, usually made from corn, used to thicken creams and lighten cakes (UK: cornflour). Custard pudding powder is a packet of flavored starch (about 37 g) that is cooked with 2 cups (500 ml) of milk and sugar into a thick pudding. In the US use about 1/3 cup (40 g) cornstarch plus 1 tsp vanilla extract per packet; only when the recipe cooks it with milk as a pudding, one box of cook-and-serve vanilla pudding mix (not instant) per packet, with the recipe's sugar reduced because the mix is sweetened. In the UK use custard powder, about 40 g per German packet (it is unsweetened, so it swaps 1:1 by weight).
3 tbspunsweetened cocoa powderSifted. Pure unsweetened cocoa powder for baking. In the US use unsweetened cocoa powder (not hot cocoa mix); in the UK, cocoa powder (not drinking chocolate).
2 sticks plus 5 tbsp300 gunsalted butterSoftened

7 tbsp100 mlmilk
6 tbsp80 gunsalted butter
9 ½ oz270 gwhite couverture chocolateWhite. Chocolate with a high cocoa butter content that melts smoothly for coating and glazing. In the US use good-quality baking chocolate bars or couverture wafers (not chocolate chips); in the UK, couverture chocolate or a good-quality cooking chocolate bar.

Line the pan with parchment paper, or grease it and dust it with flour. Preheat the oven to 350°F (170°C) convection (fan).
Whisk the oil and butter with the salt and vanilla sugar until smooth, then beat in the sugar for a few minutes.
Mix the flour and baking powder and stir them into the batter. Pour it into the pan, smooth the top and scatter the raspberries over the batter.
Bake until the cake is golden, then let it cool completely in the pan.

Mix a splash of the milk thoroughly with the cornstarch, egg yolks and cocoa powder. Bring the rest of the milk to a boil with the sugar.
Just before the milk comes to a boil, take a splash of it and stir it into the egg yolk mixture to warm it up. Repeat this two more times.
Now pour the warmed mixture into the hot milk while stirring, and keep stirring as it cooks until the cream has thickened and bubbles up once.
So that the cream does not form a skin, transfer it to a bowl while it is still hot and cover it with a lid or plastic wrap that touches the surface of the cream!
Let the covered cream come to room temperature. (IMPORTANT!)
Once the cream has reached room temperature, you can put the buttercream together: beat the butter on the highest speed for a few minutes until it is pale and creamy, then beat in the chocolate cream one spoonful at a time.
If you would like to pipe rosettes on top, set a little of the cream aside before you spread the rest smoothly over the cake.
Chill the cream-covered cake for at least 3 hours. Make the glaze in the meantime.

Heat the milk until very hot, pour it over the coarsely chopped couverture chocolate and stir thoroughly after 5 minutes. Then stir in the butter and let everything cool.
Once the cream is thoroughly chilled and the glaze is at room temperature, spread half of the glaze smoothly over the cream and let it set.
As soon as the first layer has cooled and set, spread on the second layer and make a pattern with a cake comb.
Pipe the reserved cream on top as you like with a piping bag and star tip, and finish with raspberries.

From German Pastry Chef. Full recipe with photos: germanpastrychef.com/recipes/danube-wave-sheet-cake-donauwelle/