Preheat oven to 180 C / Gas mark 4. Grease a 23cm (9 in) round cake tin. Combine the flour, baking powder and salt in a bowl. Set aside.
In the prepared tin, combine 50g butter and brown sugar. Place tin inside the preheated oven until the butter melts and begins to bubble. Remove from oven and set aside.
In a large bowl, cream together 75g butter and caster sugar until light and fluffy. Beat in the egg, then the vanilla. Beat in the flour mixture alternately with the milk, mixing just until incorporated.
Arrange plums around the edges of the prepared tin, overlapping slightly. Distribute the blueberries in the centre. Pour batter into prepared tin, covering fruit completely. Bake in the preheated oven for 40 minutes, or until a cocktail stick inserted into the centre of the cake comes out clean. Allow to cool 15 minutes before serving.