Preheat oven to 350 F. Butter a loaf pan.

In a large bowl, beat together the butter and ½ cup sugar until light and fluffy. Beat in the vanilla extract and the yolks one at a time, beating well between each addition. Beat in the milk. Add the baking powder, salt and sifted flour, beating until incorporated.

In a medium bowl, whip together the egg whites and the remaining 1/2 cup sugar until soft peaks form. Beat in 1/3rd of the whites into the mixtures, and then fold in the rest gently.

Bake for about 40-45 minutes, or until a toothpick inserted comes out clean, and the top feels springy to the touch.