This streusel cake is juicy inside and crispy outside. Instead of rhubarb and apple you can also make it with other tart fruit, such as blueberries and raspberries. Adjust the amount of sugar in the filling to your liking.
Preheat oven to 170 C / Gas 3-4. Grease a large rectangular baking dish or an extendable baking tray or line with greaseproof paper.
For the filling peel and core apple and dice. In a bowl mix apple, rhubarb, 30g sugar, cornflour and ginger. Set to one side.
For the streusel, mix butter with brown sugar, sugar, cinnamon and ginger. Add flour and mix till well combined. The mixture will be quite firm.
For the base, stir yoghurt, egg, egg yolk and vanilla till smoother. In another bowl mix flour and baking powder. Add butter and 1 spoonful of the egg mixture and beat with your electric mixer till the flour is slightly moistened, then add the remaining egg mixture in two portions and beat for 20 seconds after each addition.
If using a baking dish, spread 3/4 of the pastry base in the dish, or if using a baking tray, spread 1/2 or 2/3 over the tray. Spread filling on top.
Crumble streusel to 5mm pieces using your fingertips and scatter over the filling. Bake in the preheated oven for 50 minutes. Let cool completely before cutting.