Preheat oven to 350 degrees. Grease and flour a 15 x 10 x 1 inch jellyroll pan.
For the cake: Combine flour, baking soda, salt, sugar, eggs, and sour cream in a large bowl. Beat at medium speed until blended.
Combine 1 cup butter and water, lemon rind, lemon juice and lemon extract in a medium saucepan. Bring to a boil. Add to flour mixture, stirring well. Pour into prepared pan.
Bake at 350 degrees for 15-20 minutes or until a wooden pick inserted in the center comes out clean and cake is just starting to turn golden. Do not overbake! Cool in pan on a wire rack for about 10-20 minutes. Cake should still be warm when you frost it.
For the frosting: Combine remaining 1/2 cup of butter, milk and 1 teaspoon lemon extract in saucepan; bring to a boil. Remove from heat. Gradually add powdered sugar, stirring until mixture is spreading consistency. Immediately spread frosting on top of warm cake. Let frosting set.
Notes
I feel like this cake is better the second day because the frosting makes the cake even more moist.