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Preheat the oven to 350°F. Liberally grease a 9x13 inch pan with non-stick baking spray.
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In a bowl whisk together the cake flour, baking powder, baking soda, and salt together. Set aside.
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Using a handheld or stand mixer, beat the butter on high speed until light and creamy.
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Add the sugar and beat on high speed for 2 minutes until creamed together, scraping down sides of bowl.
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Add the egg whites. Beat on high speed until combined, about 2 minutes.
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Then beat in the sour cream and vanilla extract. Scrape down the sides and up the bottom of the bowl as needed.
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With the mixer on low speed, add the dry ingredients until just incorporated.
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With the mixer still running on low, slowly pour in the milk until combined.
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Do not overmix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl.
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The batter will be slightly thick.
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Pour the batter into a greased and lightly floured 9×13 pan and bake for 40 minutes or until a toothpick inserted in the center comes out clean.
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Cool completely before frosting and decorating.