Set rack at the middle level in the oven and preheat to 350 degrees.
Butter the bottom of two 9-inch round or one 13 by 9 by 2-inch pan. Line bottom with parchment or waxed paper.
In a large bowl, beat butter and sugar for about 5 minutes, until light and fluffy. Stir together flour, baking powder and salt. Set aside. Combine egg whites, milk and vanilla extract. Add 1/3 of the flour mixture to the butter mixture then add half the milk mixture. Continue to alternate beginning and ending with flour mixture. Scape the bowl and beater often.
Pour the batter into prepared pan(s) and smooth top with metal spatula. Bake cake(s) about 25 to 30 minutes, or until a toothpick inserted in the center emerges clean.
Cool in pan on rack for 5 minutes, then turn out onto a rack, remove paper and let cool completely.
Note: For a classic yellow cake- Substitute 3 large eggs and 1 yolk for the egg whites.
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