Grease two 10 x 3″ round pans. Preheat the oven to 350. Have all ingredients at room temperature.
WBH White Butter Cake
In the bowl of an electric mixer, beat at high speed until light and fluffy:
6 oz (1 1/2 sticks) unsalted butter
2 cups sugar
1 1/2 tsp. salt
On a piece of wax paper, sift together:
3 1/3 cups cake flour
1 1/2 teaspoons baking powder
3/4 teaspoon cream of tartar
Add the dry ingredients to the butter mixture alternately with:
1 1/2 cups milk
3/4 teaspoon pure vanilla extract
In a separate bowl beat until foamy:
3/4 cup egg whites (6 to 8 large egg whites)
Continue beating at high speed while gradually adding:
1/2 cup sugar
When stiff peaks form, fold the whites into the cake batter with a rubber spatula. Blend just until the whites are evenly distributed.
Pour 3 1/2 cups of the batter into one prepared pan and the reamining batter into the other. Bake the less full pan for 20-25 minutes and the fuller pan for 30 to 35 minutes, or until a cake tester comes out clean. Cool the cakes on a wire rack for 15 to 20 minutes before turning them out our their pans.