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Preheat oven to 350 degrees and line two 12-cup muffin pans (or a mini muffin tin).
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In a large bowl, cream butter until light and fluffy, about 1-2 minutes. Gradually add in sugar, 1/2 cup at a time, beating until well combined.
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In a separate bowl, sift together flour, cornstarch, baking powder, and salt. Set aside.
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In a small bowl, mix together milk, lemon juice, vanilla extract, and Greek yogurt.
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Alternately add dry and wet ingredients into the creamed butter mixture, beginning and ending with the dry ingredients. Stir until just combined.
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Beat egg white until they form stiff peaks. Gently fold egg whites into the cake batter.
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Pour batter into cake pans, dividing evenly between the two. Bake for 16-17 minutes, or until a cake tester comes out clean. For mini cupcakes, bake for 8-9 minutes, or until a cake tester comes out clean.