unsalted butter, softened
- Butter two 9-inch diameter by 1 1/2-inch deep layer pans or 1 (13 by 9 by 2-inch) pan.
- Lined bottoms with parchment or waxed paper.
- Set a rack at the middle level of the oven and preheat to 350 degrees.
- In a large mixing 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 butter mixture then add half the milk mixture.
- Continue to alternate beginning and ending with flour mixture.
- Scrape bowl and beater often.
- Pour batter into prepared pan (s) and smooth top with a metal spatula.
- Bake cake (s) about 25 to 30 minutes, or until a toothpick inserted in the center emerges clean.
- Cool in pan on a rack for 5 minutes, then turn out onto a rack, remove paper and let cool completely.