Preheat oven to 180 C / Gas 4. Grease and flour a 20cm square tin.
Cream butter or margarine and 100g sugar until fluffy. Add salt and vanilla. Separate eggs and reserve the whites. Add egg yolks to the sugar mixture; beat until creamy.
Combine 200g flour and baking powder; add alternately with milk to egg yolk mixture. Coat berries with 1 tablespoon flour and add to mixture.
In a separate bowl, beat whites until soft peaks form. Add 4 tablespoons of sugar, 1 tablespoon at a time, and beat until stiff peaks form. Fold egg whites into mixture. Pour into prepared tin. Sprinkle top with remaining 1 tablespoon sugar.
Bake for 50 minutes, or until cake tests done
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