Preheat the oven to 180 C / Gas mark 4. Grease a 23cm square baking dish.
Place the rhubarb and strawberries into a bowl. Stir in 1/2 of the sugar until fruit is coated. Pour into the prepared dish and spread out to cover the bottom. In a medium bowl, beat the remaining sugar with the butter until light and fluffy, about 3 minutes. Mix in the egg, orange zest and vanilla. Combine the flour, baking powder, bicarbonate of soda and salt; stir into the batter, alternating with the buttermilk. Pour the batter over the fruit in the dish.
Bake in the preheated oven until a skewer inserted into the centre comes out clean, about 55 minutes.
This is more of a dessert than a cake, but that said my whole family loved it. The children had it with icecream but I think it is nicer with cream or creme fraiche. My husband says this is now his favourite ever dessert! - 08 May 2011