1 cup (2 sticks) unsalted butter, room temperature, plus more for pan
2 cups cake flour, (not self-rising)
1 3/4 cups all-purpose flour
2 teaspoons baking powder
3 1/4 cups sugar
3/4 teaspoon salt
1 tablespoon pure vanilla extract
1 cup large egg whites (about 8), room temperature
1/2 cups milk, room temperature
Preheat oven to 325 degrees. Butter a 9-by-13-inch cake pan; line bottom with parchment paper. Sift together flours and baking powder; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and sugar until light and fluffy. Add salt and vanilla, beating until combined. With mixer on low speed, gradually add egg whites, scraping down sides of bowl as necessary. Add reserved flour mixture in 2 parts, alternating with the milk, and scraping down sides of bowl between each addition. Mix until combined. Increase speed to medium-high for 20 seconds. Transfer to prepared pan. Bake until a cake tester inserted in the center comes out clean, about 45 minutes. Transfer to a wire rack to cool completely.