Rahmkuchen (yeasted sour cream custard cake)

Method

1. Whisk the flour, sugar, salt and yeast together, then add the lukewarm milk, melted butter and egg. Knead by hand or with a dough hook for 8 minutes, until smooth and elastic.
2. Cover and prove in a warm spot for 60 minutes, until roughly doubled.
3. Press the risen dough out onto a lined baking sheet, about 30 by 20 cm, going right to the edges. Prick it a few times with a fork and let it rest, covered, for 15 minutes.
4. Whisk the sour cream, eggs, sugar, cornstarch and vanilla sugar together until smooth, then pour evenly over the dough.
5. Bake at 190°C for 35 to 40 minutes, until the custard is set at the edges and still wobbles gently as one piece in the centre. Tent loosely with foil if the top browns before the centre sets.
6. Cool in the tray for 20 minutes, then chill at least 1 hour before cutting into squares; the custard needs that time to firm up enough to hold a clean cut.

Notes

• The custard should still wobble when it comes out. Baked until completely firm, it will have overbaked by the time it cools, and the texture turns rubbery rather than soft.
• A soft, slightly sticky dough is correct here. Adding extra flour to make it easier to handle gives a denser cake than this tray-bake tradition calls for.
• This is deliberately plain, and that is the point; it is a good base to add fruit, cinnamon, or a streusel topping to if a plainer cake does not suit the occasion.
• It keeps for 3 days in the fridge, covered, and is very good cold straight from the fridge.

Ingredient substitutions

• Sour cream can be crème fraîche for a slightly milder tang, or a mix of sour cream and quark for a firmer set.
• Instant yeast can be swapped for fresh yeast at roughly three times the weight, crumbled into the lukewarm milk with a pinch of the sugar and left for 10 minutes until foamy before mixing in.
• A handful of raisins, soaked in warm water or rum and scattered over the dough before the custard goes on, is a common traditional addition.
• A light dusting of cinnamon over the custard before baking is a simple way to vary the flavour without changing the technique.