Preheat oven 325℉ .In a large bowl, add flour, sugar, salt and baking powder. Mix together.Add butter, milk and lemon juice. Butter should be cooled, not hot. Mix together.Add the eggs and mix together with hand mixer.Add lemon (or vanilla) extract and zest, mixing together. Place into 9x13 pan (spray with non-stick spray). You can also use 2 round pans or make cupcakes.Bake for 30-40 minutes until a toothpick check comes out clean.
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Notes
If using salted butter, reduce amount of salt by half.