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Grease and flour (3) 8” round cake pans; set aside.
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Preheat oven to 325°F.
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In a medium bowl, sift together the flour, baking powder and salt; set aside.
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In a separate bowl, whisk together the milk, sour cream, vanilla extract, lemon juice and lemon zest; set aside.
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Using a countertop mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy (~3-4 minutes on medium-high speed).
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Add ~½ of the flour mixture to the bowl; mix on low speed until well combined.
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Add the milk mixture to the bowl. Mix on low until well combined.
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Add the remaining flour mixture; mix on low until well combined. (Tip: Don’t forget to scrape down the sides of the bowl to ensure all ingredients are fully mixed.)
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Fold the blueberries into the cake batter.
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In a separate bowl, beat egg whites on medium speed until stiff peaks form. Carefully fold the egg whites into the cake batter until fully mixed.
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Divide the batter evenly between the 3 cake pans. Bake at 325°F for 38-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.
