Danube Wave Cake with Sour Cherries, Custard Cream, and Glaze

Preheat the oven to 180°C on the conventional (no-fan) setting. Line the bottom and sides of a rectangular metal pan with internal dimensions of 24 × 30 cm and a height of at least 6 cm with parchment paper, without extra fat. Drain the sour cherries in a sieve for about 10 minutes, discard the packing liquid, and weigh out 500 g fruit; check that no pits remain. Prepare two bowls for dividing the batter. Divide the butter into two 200 g portions: both should be soft but not melted; the eggs and 60 ml milk for the cake should also be at room temperature.

Beat 200 g softened butter with 180 g sugar for about 5 minutes until noticeably paler and airy. Add 4 eggs one at a time, mixing briefly after each until combined. Sift together 250 g flour and 8 g baking powder, add to the mixture, and mix on low speed just until the dry flour disappears. Do not overmix after adding the flour.

Divide the pale batter into two equal portions in separate bowls. Add 30 ml milk to one, mix briefly, and spread over the bottom of the pan. Add 20 g sifted cocoa and the remaining 30 ml cake milk to the other, mix briefly, then place small dollops over the pale base and gently level. Arrange all 500 g drained sour cherries evenly over the cocoa layer and press in lightly, not all the way to the bottom. Keep the pale and dark batters as separate layers; do not swirl them with a fork into a marble pattern.

Bake on the middle rack for about 40–45 minutes. The top should be springy, and a skewer inserted into the cake between the cherries should come out without wet, raw batter; if it is wet, bake for about 5 more minutes. After removing it from the oven, place the pan on a rack and leave for about 90 minutes until the cake is completely cool. Cook the custard as described in the next step while the cake bakes, so both can cool at the same time.

From the 500 ml milk for the cooked custard, pour off 100 ml and mix smoothly with 40 g cornstarch. In a saucepan with a capacity of about 1.5 l, bring the remaining 400 ml milk, 80 g sugar, and 5 ml vanilla extract to a boil. Stir the slurry again and pour it in, whisking vigorously; once it thickens and bubbles appear, cook over low heat for another 2 minutes, stirring constantly across the bottom. Transfer all the custard to a wide container in a layer about 2–3 cm deep, cover its surface directly with plastic wrap suitable for hot food, and cool for about 90 minutes. It should be completely cool but not ice-cold, at a similar temperature to the softened 200 g butter for the cream; cook the custard during baking and cool it alongside the cake.

Check that the custard and softened butter do not feel noticeably different in temperature; do not add warm custard to the butter. Beat the remaining 200 g softened butter with a mixer for about 5 minutes until airy. Add all the cooled custard one spoonful at a time on medium speed, mixing each addition until combined before adding the next. The finished cream should be smooth, uniform, and spreadable, without butter lumps or separated liquid.

Spread all the cream evenly over the completely cool cake in the pan, reaching the corners. Refrigerate immediately for 1 hour so the cream sets before the glaze is applied. Toward the end of this chilling time, prepare the glaze and start cooling it as described in the next step.

Chop 150 g chocolate and place in a bowl. Heat 100 ml cream in a small saucepan until small bubbles appear around the edges, pour over the chocolate, wait 2 minutes, and stir until smooth. Leave for about 25–30 minutes, stirring a few times, until the glaze feels cool to the touch but still flows freely from a spoon. Start about 40 minutes before the hour of cream chilling ends so the glaze cools at the same time; do not apply it hot or warm.

Remove the pan from the refrigerator after the full hour of cream chilling. Spread all the cool, still-fluid glaze evenly over the cream with light pressure from the spatula. Do not mix the chocolate into the pale cream, and do not add liquids to the glaze. Return the pan to the refrigerator immediately.

Chill for at least 2 more hours until the glaze sets and the cream holds an even layer during slicing. Divide the shorter side into three sections and the longer side into four, making 12 pieces about 8 × 7.5 cm. Cut with a sharp knife, wiping it between cuts, and serve. Return the remaining portions to the refrigerator immediately.