Grease and flour 20cm square cake tin. Preheat oven to 180 C. Mix flour, baking powder and salt. In another bowl, cream the butter and sugar until fluffy, mix in the beaten egg yolks and vanilla essense. Stir in 1/3 of flour mix, then 1/2 the milk, 1/3 flour, rest of milk and then the rest of the flour mix. Mix well each time. Beat the egg whites until stiff then fold into the cake mix. Fold in blueberries and pour into cake tin. Sprinkle top with an additional tablespoon sugar and bake for 45-50 minutes until cake tester comes out clean. Dust with icing sugar before serving.