Zwetschgendatschi (Bavarian Plum Sheet Cake)

Method

1. 1Make the dough. Warm the milk until it feels comfortably warm on your wrist, not hot, and stir in the yeast with a pinch of the sugar; if using instant yeast, whisk it directly into the flour instead. Combine the flour, remaining sugar, and salt in a large bowl, add the milk mixture, the egg, and the soft butter, and knead by hand or with a dough hook for 8 to 10 minutes. The dough is ready when it turns smooth, slightly glossy, and pulls cleanly away from the sides of the bowl; if it still looks shaggy after 10 minutes, knead in another tablespoon of milk.
2. 2Let the dough rise, then shape it. Cover the bowl with a damp towel and set it in a warm spot for 45 to 60 minutes, until it has visibly puffed up and springs back slowly when you press a finger into it. Turn it out onto a floured 30x40 cm rimmed baking sheet lined with parchment, and press and stretch it with your fingers to fill the sheet in an even layer, building a slightly raised rim at the edges to hold the fruit and its juice.
3. 3Prepare the plums. Wash and halve the plums, then twist out the pits. Working with the cut side up, score each half lengthwise almost all the way through, without cutting it apart, and press gently so it fans open like a small accordion. Zwetschgen are the right plum for this: their firm, freestone flesh holds its shape and releases far less juice in the oven than round, clingstone plum varieties, which tend to collapse into mush.
4. 4Cover the dough with fruit. Arrange the plum halves skin-side down in tight, slightly overlapping rows across the entire sheet, packing them edge to edge so almost no bare dough shows through. Sprinkle the sugar and cinnamon evenly over the top. Packing them this tightly matters: the fruit shrinks as it bakes, and a loose arrangement leaves gaps that dry out and burn before the middle of the cake is done.
5. 5Make the streusel, if using. Rub the flour, sugar, soft butter, vanilla extract, and cinnamon together with your fingertips until you get coarse, pea-sized crumbs; do not overwork it into a paste. Scatter the streusel evenly over the plums. It is optional, but it does real work: it soaks up the juice the plums release as they bake, which keeps the dough underneath from turning soggy.
6. 6Bake. Preheat the oven to 190°C top and bottom heat (170°C fan) while the dough finishes its first rise, if you have not already. Bake the sheet in the middle of the oven for 30 to 35 minutes, until the exposed dough at the edges is deep golden brown and the plum juices are bubbling gently around the streusel.
7. 7Cool and serve. Let the cake cool on the sheet for at least 15 minutes before cutting it into 12 squares; it firms up as it cools, and cutting it too soon lets the warm fruit juice run everywhere. Serve it warm or at room temperature with a spoonful of lightly whipped cream on the side.