Preheat oven to 375°F. Grease a 13 x 18 inch sheet pan and line with parchment paper.
In a medium bowl, combine the flour, salt, baking soda, and zest and set aside. In the bowl of an electric mixture, beat together the butter and sugar until light and fluffy, about 3 minutes. Add the eggs one at a time, mixing after each. Add the vanilla. Add the flour mixture in 2 addition, mixing until just combined.
Press the cookie dough into the sheet pan, using a greased spatula or parchment paper to spread it evenly. Bake for 10-15 minutes until lightly golden and a toothpick inserted into the middle comes out clean. Cool completely. Frost with lemon buttercream frosting.
For the buttercream:
In the bowl of an electric mixer, cream together the butter, powdered sugar, vanilla and salt and mix until combined. Add the lemon juice 1 tablespoon at a time until desired consistency is reached. Beat on medium high speed for a few minutes until smooth.
Spread onto cooled cookies. Slice into bars and serve.
Notes
Note: If you want a stronger lemon flavor in the buttercream, add 1 tablespoon lemon zest.