Preheat oven to 350F. Place the flour, baking powder and salt in a bowl, and mix until combined. Place the egg whites in a separate bowl, and beat to stiff peaks.
Melt the butter in a bowl in the microwave (approx. 1 minute). Add the sugar, and mix until homogeneous. Add the egg yolks one by one, mixing after each addition.
Add the flour and beaten egg whites in stages, starting with the egg whites and finishing with the flour (for example, you can add half of the egg whites, then half of the flour, then the other half egg whites, and half of the flour). Add the Nielsen-Massey Madagascar Bourbon Pure Vanilla Extract. Mix until just combined.
Pour the batter into a buttered and floured 9inx9in baking pan. Wash and dry the blueberries with a paper towel, then place them in a bowl with about 2 tbsp flour. Lightly coat the blueberries with flour, adding more flour if necessary. Distribute the blueberries on top of the batter uniformly.
Bake for approximately 45 minutes or until golden-brown. You can check for doneness by inserting a toothpick into the center of the cake. The cake is done if the toothpick comes out clean and dry.
Take the cake out of the oven and let it cool down completely. Cut into squares and serve with whipped cream on top or extra blueberries on the side - or both!