Heat the oven to 350 F.
Rub the butter into the flour and sugar until it's the consistency of breadcrumbs.
Mix in the beaten egg. Stir in the fruit gently so as not to break it; it's best to use your hands. The mixture will look dry but don't worry; the end result is very moist.
Turn into an ovenproof glass or china dish (the acidity of the fruit may react with metal and taint the cake).
Bake for 45-50 minutes.
Turn out carefully and sprinkle with super fine sugar. Eat at room temperature.