Donauwelle (Danube Wave Cherry Sheet Cake)

Instructions

1. Make the pudding for the buttercream first so it can cool: whisk the cornstarch and 80 g granulated sugar with 100 ml of the 400 ml whole milk until smooth. Bring the remaining 300 ml milk to a boil, whisk in the cornstarch mixture, and cook, whisking, until thick and bubbling for 1 minute. Stir in the 5 ml vanilla extract, press plastic wrap onto the surface, and let it cool to room temperature, about 1 1/2 hours.
2. Heat the oven to 350°F (175°C). Butter the baking pan and line the base with parchment.
3. Make the cake: beat the 200 g softened butter, 200 g granulated sugar and 5 ml vanilla extract until pale and fluffy, about 3 minutes. Beat in the eggs one at a time. Whisk the all-purpose flour, baking powder and fine salt together and fold them in, alternating with 45 ml of the 60 ml whole milk, until smooth.
4. Spread half the batter evenly in the pan. Stir the unsweetened cocoa powder and the remaining 15 ml milk into the other half, then spread the cocoa batter over the light layer.
5. Scatter the jarred sour cherries over the top and press them lightly into the batter. Bake until risen and a skewer inserted between the cherries comes out clean, 30 to 35 minutes. Cool completely in the pan on a rack, about 1 1/2 hours.
6. Make the buttercream: beat the 200 g softened butter with a mixer until very pale and creamy, about 5 minutes. Add the cooled pudding a spoonful at a time, beating well after each, until smooth and fluffy. The butter and pudding must both be at room temperature or the buttercream will curdle.
7. Spread the buttercream evenly over the cooled cake and refrigerate until firm, about 45 minutes.
8. Make the chocolate top: melt the dark chocolate and 25 g butter in a bowl over barely simmering water, stirring until smooth, then cool 5 minutes. Spread thinly over the chilled buttercream. When it just begins to set, after about 10 minutes, drag a cake comb or fork through it in wavy lines.
9. Refrigerate until the chocolate is set, about 30 minutes. Cut into 15 pieces with a knife dipped in hot water and wiped between cuts.