Rhabarber-Streuselkuchen (rhubarb streusel cake with vanilla custard)

Method

1. Rub the cold butter into the flour, sugar, baking powder and salt until it looks like coarse crumbs, then work in the egg just until a dough forms. Press into the base of a 26 cm springform, about 2 cm up the sides, and chill for 20 minutes.
2. Make the streusel: rub the cold butter into the flour and sugar with your fingertips until it forms coarse, pea-sized crumbs. Chill until needed.
3. Toss the rhubarb with the 30 g sugar and scatter it evenly over the chilled base.
4. Whisk the egg yolks, 40 g sugar and cornstarch into a little of the cold milk until smooth. Heat the rest of the milk until steaming, then whisk it in. Return to the pan and cook over low heat, whisking constantly, until it thickens to a pourable custard, about 4 minutes. Stir in the vanilla.
5. Pour the warm custard evenly over the rhubarb, then scatter the streusel over the top in an even layer.
6. Bake at 180°C for 40 to 45 minutes, until the streusel is solidly golden and the custard looks set rather than liquid.
7. Cool in the tin for 20 minutes, then release the sides and cool completely on a rack before slicing.

Method in the Thermomix

Written for a TM5 or TM6, 2.2 litre bowl. This covers the custard; the dough, streusel and baking are the same as the hand method.

1. Milk, egg yolks, 40 g sugar and cornstarch into the bowl. Cook 6 min / 90 °C / speed 3, until thickened to a pourable custard.
2. Stir in the vanilla. Mix 5 sec / speed 2. Pour over the rhubarb and continue as in the hand method.

Notes

• The custard is what keeps this from being a wet, soupy cake. Rhubarb releases a lot of liquid as it bakes, and the custard sets around it rather than letting that liquid soak straight into the streusel.
• Do not skip tossing the rhubarb with sugar first. Rhubarb on its own is sharply sour, and a few minutes with sugar before it goes into the cake takes the edge off.
• Keep the streusel cold until it goes into the oven, for the same reason as any other streusel cake; warm butter melts into a sheet rather than baking into crumbs.
• It keeps for 3 days in the fridge, covered, and is best the day it is baked while the streusel is still crisp.

Ingredient substitutions

• Rhubarb can be part-replaced with strawberries, about a third of the total weight, for a classic rhubarb-strawberry pairing.
• A dusting of ground ginger in with the streusel is a good match for rhubarb’s sharpness.
• For a dairy-free version, use oat milk for the custard and a plant-based butter throughout, though the custard sets slightly softer.
• Frozen rhubarb works if thawed and drained of excess liquid first; skipping the draining leaves the custard too wet to set properly.