Preheat the oven to 180C. Grease and flour a 20x30cm baking dish.
In a large bowl, stir together the flour and sugar. Cut in butter using a pastry blender or a fork until pea-sized. Measure out 3/4 cup of the mixture and set aside. Stir baking powder and salt into the remaining mixture then mix in the egg yolks and milk using an electric mixer on low speed for 3 minutes.
In a large glass or metal bowl, whip egg whites just until stiff peaks begin to form. Fold egg whites into the batter. Spread batter evenly into the prepared pan. Sprinkle blueberries over the batter, and sprinkle the reserved crumb mixture over the top.
Bake for 40 to 50 minutes in the preheated oven, or until the cake springs back when lightly touched in the center.