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Position oven racks in middle of oven. Preheat oven to 350°F/180°. Line 24 cupcake wells with fluted paper cups; set aside.
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Whisk together the milk and lemon juice in a mixing bowl and allow to sit for about 5 minutes to thicken. Add egg whites and whole egg to thickened milk and whisk until well combined; set aside.
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Whisk together the flour, cornstarch, baking powder, baking soda and salt in a mixing bowl to aerate and combine; set aside.
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Beat butter with an electric mixer on medium-high speed in a large mixing bowl until very smooth and creamy, about 3 minutes, scraping down the bowl once or twice. (If using a stand mixer, use the flat paddle attachment). Beat in the sugar gradually and continue beating until fluffy, about 4 minutes, again scraping down the bowl as needed. Beat in vanilla.
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Alternately add the wet mixture and dry mix to the butter/sugar mixture in 3 batches, beating well until smooth and combined. Evenly divide batter among fluted paper cups. Bake cupcakes for about 15 to 20 minutes, rotating the pans front to back once during baking. When done the tops should spring back when lightly pressed and a toothpick inserted in the center should show a few moist crumbs clinging. Place pans on a cooling rack for 5 minutes, then unmold cupcakes and cool upright. (If you want to freeze cupcakes for future use, place in zip top bags with air removed and freeze for up to 1 month. Defrost in refrigerator).