Make the Cake
Preheat the oven to 350F degrees.
Grease and flour a 9x13 inch baking pan.
In a medium bowl sift together the flour, baking powder, baking soda and salt.
In a large bowl beat together the butter and sugar until fluffy.
Then beat in the eggs and vanilla, followed by the sour cream.
Turn the mixer down to low speed and carefully beat in the flour mixture followed by the buttermilk.
Then turn off the mixer and stir in the lemon zest and lemon juice using a large rubber spatula or wooden spoon.
Pour the batter into the prepared pan and bake for 27-33 minutes, or until an inserted toothpick comes out clean.
Allow to cool fully before frosting
Make the Cream Cheese Frosting
In a large bowl, beat together the butter and cream cheese until evenly mixed together.
Then slowly beat in the powdered sugar, turning off the mixer and scraping down the sides of the bowl as necessary.
Finally, beat in the lemon juice and salt and a teaspoon of milk if needed.
Frost the cooled cake using a flat edge knife.
For easier frosting, you can also chill the cake in the fridge or freezer prior to frosting. Cut into slices and serve.