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Preheat the oven to 350°F. Lightly grease two 8" round pans that are at least 2" deep, and line them with parchment or silicone liners; grease the parchment or liners. If you’ll be using cake strips to ensure an even rise, soak 2 strips in cold water.
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To make the batter: In the bowl of a stand mixer, whisk together the flour, cornstarch, sugar, salt, baking powder, baking soda, and lemon juice powder.
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Use the flat beater attachment to mix the butter into the dry ingredients, beating on low speed until the mixture is like coarse sand, 1 to 2 minutes.
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In a separate bowl or large measuring cup, whisk the egg whites until frothy. Stir in the buttermilk, oil, extracts, and vinegar.
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With the mixer running, gradually add the liquid ingredients to the butter/flour mixture. Once all the liquid has been added, scrape the bottom and sides of the bowl. Beat the batter for 2 minutes at medium speed (speed 4 on a KitchenAid mixer); scrape the bottom and sides of the bowl again and mix briefly to incorporate any sticky residue, if necessary.
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Divide the batter between the prepared pans (about 720g per pan); wrap the soaked cake strips around the sides of the pans, if you’re using them.
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To bake the cakes: Bake for 35 to 40 minutes, until the cakes are golden brown and starting to pull away from the sides of the pan; a toothpick or thin knife inserted into the center should come out clean, or with only a few crumbs clinging, and the internal temperature at the center should be about 204°F when tested with a digital thermometer.
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Remove the cakes from the oven, and after 5 minutes loosen their edges with a small offset spatula or a dull knife. After another 5 minutes, turn them out onto a rack to cool completely.
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Fill and frost the white velvet cake layers with Cream Cheese Frosting, Ermine Icing, or the frosting of your choice. (See our video, how to assemble and decorate a layer cake, for details.) For extra-moist cake layers, apply a soak as you assemble.
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Storage information: Store the unfrosted white velvet cake layers, well covered, for up to 3 days at room temperature. Depending on which frosting you use, store the cake at room temperature or securely wrapped in the refrigerator for up to 2 days.