Preheat the oven to 180 C / Gas 4. Grease a 23cm loose-bottomed cake tin.
Beat butter in a large bowl with an electric mixer until creamy. Add sugar and salt gradually and mix in well. Add eggs one at a time, beating well after each addition.
Combine flour and baking powder; sift and quickly beat into the creamed mixture on low speed. Thin mixture with a little milk if needed. Pour cake mixture into the prepared tin. Arrange nectarines and blackberries on top in a decorative pattern.
Bake in the preheated oven until a skewer inserted into the centre comes out clean, about 45 minutes. Dust with icing sugar and cool on a wire rack for 5 minutes. Run a table knife around the edges to loosen. Turn cake out carefully onto a serving plate. Let cool completely.