1.Preheat oven to 325°F. 2.In a bowl, mix together flour, salt, and baking soda. 3.Using a hand mixer (or stand mixer), cream butter with sugar and lemon zest, until light and fluffy (about 8 mins). Add the sour cream, mix until combined. Then beat in the eggs and egg yolk 1 at a time. Add the lemon juice, mix until well combined.