Preheat the oven to 325℉. Grease and flour one 5x9 inch loaf pan.
Whisk together the flour, baking powder and salt in small bowl. Set aside.
In medium bowl, beat the egg whites until stiff. Set aside.
Rinse off the beaters and use an electric mixer to cream the butter with the sugar in a large bowl. Beat until creamy and light. Mix in the egg yolks, one at a time.
Gradually blend in the dry mixture. Add the vanilla. Fold the egg whites into the batter.
Pour the batter into the prepared pan and bake for one hour. Test doneness with a toothpick inserted into the center. It should come out clean.